BIOLOGY 
LIBRARY 


'HISTORY 

OF 

THE    HUMAN    BODY 


BY 

HARRIS  HAWTHORNE  WILDER 

ii 

Professor  of  Zoology  in  Smith  College 


NEW  YORK 

HENRY  HOLT  AND  COMPANY 

1909 


LtBRAHY 
G 


COPYRIGHT,  1909, 

BY 
HENRY  HOLT  AND  COMPANY 


Seinem  Lehrer  und  Freunde 
<£*lj*tm-Ijufrat  Unhurt  WwterBljrtm 

wird  dieses  Buck 
in  Liebe  und  Dankbarkeit  gewidmet. 

DER  VERFASSER. 


257867 


PREFACE 

THIS  book  has  a  twofold  purpose:  first,  to  present  the  re- 
sults of  modern  anatomical  and  embryological  research  rela- 
tive to  the  human  structure  in  a  form  accessible  to  the  general 
student,  and,  secondly,  to  furnish  students  of  technical  human 
anatomy  with  a  basis  upon  which  to  rest  their  knowledge  of 
details. 

Regarding  the  first  of  these  purposes,  it  may  be  said  that, 
while  many  of  the  phases  of  the  doctrine  of  evolution  have 
been  thoroughly  exploited,  and  their  general  teaching  has  be- 
come the  property  of  the  general  scholar,  the  contribution 
to  thought  furnished  by  anatomy  has  been  considered  of  too 
technical  a  character  for  popular  presentation.  It  is  true  that 
this  science  necessarily  rests  upon  a  material  basis,  and  in- 
volves a  mass  of  extremely  intricate  details,  and  it  is  also  true 
that  a  more  or  less  complete  knowledge  of  these  is  absolutely 
necessary  before  the  contribution  of  this  science  to  evolution- 
ary thought  can  be  appreciated ;  but  in  these  respects  anatomy 
does  not  differ  from  other  branches  of  natural  science,  the 
essential  teachings  of  which  are  already  a  matter  of  general 
knowledge.  If,  then,  the  technical  difficulties  have  been  sur- 
mounted in  the  case  of  Geology,  Astronomy  and  general 
Zoology,  it  is  not  too  much  to  hope  that  in  the  course  of  the 
next  few  years  the  mission  of  Anatomy  may  also  become  gen- 
erally known,  especially  since  its  results  touch  human  interests 
more  closely  than  do  those  of  any  of  the  kindred  sciences. 

Concerning  the  second  purpose,  that  of  assisting  in  the 
technical  study  of 'human  anatomy,  it  is  hardly  necessary  to 
present  an  argument,  since  the  great  advantages  of  studying 
human  anatomy  in  connection  with  both  comparative  anatomy 
and  embryology  are  patent  to  all  who  have  employed  this 
method.  While  there  are  still  a  few  human  anatomists  who 
present  the  old  argument  that  the  science  is  too  full  of  detail 
already  to  allow  the  assumption  of  additional  facts,  the  ex- 


VI 


PREFACE 


perience  of  everyone  who  has  learned  the  parts  of  some  com- 
plicated organ  like  the  brain,  by  the  old  method,  and  has  had 
it  later  elucidated  by  the  new,  is  a  sufficient  refutation  of 
such  a  position.  It  takes  but  a  little  experience  with  anatomy, 
as  taught  by  the  modern  comparative  method,  to  see  that  this 
latter  furnishes  a  rational  basis  for  an  absolute  knowledge  of 
the  fundamental  relationships,  while  the  old  method  is  largely 
an  intricate  system  of  mnemonics.  A  student  of  the  older  anat- 
omy must  needs  remember  arbitrarily  that  two  given  parts 
are  related  in  a  certain  way  and  not  in  the  reverse  way,  and 
if  his  memory  is  inadequate  to  the  task  he  has  nothing  to 
save  him,  while  a  student,  furnished  with  a  morphological 
basis  for  his  knowledge  and  able  to  refer  the  parts  back  to 
a  time  in  which  they  were  in  a  much  simpler  condition,  will 
know  that  they  must  be  related  in  a  certain  definite  way,  and 
cannot  be  otherwise  arranged. 

The  present  work  has  especially  the  needs  of  the  medical 
student  in  mind,  since  it  is  not  a  general  comparative  anatomy, 
but,  as  its  title  signifies,  a  "  history  of  the  human  body,"  in 
which  the  structure  of  the  lower  vertebrates  is  expounded 
only  so  far  as  is  needed  to  throw  light  upon  the  relations 
found  in  Man.  Thus  the  lines  that  do  not  lead  in  this  direc- 
tion, but  represent  specialized  side-branches,  like  those  of 
birds  or  snakes,  are  barely  touched  upon,  other  than  as  illus- 
trations of  principles  similar  to  those  under  consideration, 
although  certain  exceptional  modes  of  development  or  eccen- 
tric specializations  are  often  mentioned  on  account  of  their 
general  interest. 

The  technical  terms  of  human  anatomy  employed  in  this 
work  conform  in  general  to  the  list  prepared  by  the  Basle 
Anatomical  Nomenclature  (BNA),  but  in  cases  where  these 
terms  differ  widely  from  those  in  common  use  in  America  the 
latter  are  placed  in  brackets  after  the  BNA  term.  In  cases 
where  the  BNA  nomenclature  is  not  in  accord  with  morpho- 
logical principles,  these  terms  are  rejected,  but  are  indicated 
in  brackets  or  otherwise.  Of  these,  the  most  important  are 
the  following: 


PREFACE  vii 

1.  In  the  case  of  the  bones  of  the  carpus  and  tarsus.     For 
these  the  BNA  nomenclature  employs  the  terms  used  on  the 
Continent,  and  especially  Germany  (e.g.,  triquetrum,  multan- 
guhim  majuSj  etc.),  instead  of  those  to  which  the  Americans 
and  English  are  accustomed.  The  synonomy  of  these  terms  is 
presented  in  the  form  of  a  table,  but  as  both  sets  are  purely 
arbitrary  and  describe  the  shapes  and  relative  sizes  as  found 
in  Man  alone,  there  seems  no  reason  why  one  should  be  pre- 
ferred to  the  other,  or,  indeed,  why  either  should  be  longer 
perpetuated,  in  preference  to  the  simple  system  employed  by 
comparative  morphologists. 

2.  In  several  cases  in  which  terms  of  orientation  are  still 
employed  with  reference  to  Man  in  a  standing  position  (e.g., 
superior  and  inferior  instead  of  anterior  and  posterior;  an- 
terior and  posterior  instead  of  ventral  and  dorsal).   Thus,  in 
the  case  of  the  columns  of  the  spinal  cord,  it  is  thought  best 
to  reject  the  BNA  terms  posterior  and  anterior  in  favor  of 
the  more  natural  dorsal  and  ventral,  as  employed  in  the  case 
of  all  other  animals.    In  the  same  way  the  two  vence  cava  are 
referred  to  as  anterior  and  posterior  instead  of  superior  and 
inferior. 

3.  In  the  case  of  the  pads  of  the  palm  and  sole.    Here  the 
principle  involved  is  one  of  use  rather  than  position,  and  the 
point  at  issue  depends  upon  the  true  function  of  these  parts. 
The  two  views  held  at  present  are  (i)  that  their  function  is 
tactile,  and  (2)  that  it  is  mechanical,  preventing  the  tendency 
to  slip  by  presenting  a  surface  covered  by  ridges,    [cf.  Chap- 
ter IV.]     The  BNA  term  for  these  pads  is  toruli  tactiles,  a 
term  which  does  not  accord  with  the  view  expressed  here. 

In  a  few  cases  the  adoption  of  the  new  nomenclature  in- 
volves changes  in  well-established  terms ;  for  example,  ductus 
[vas]  deferent,  stratum  germinativum  [mucosum],  and  renal 
[Malpighian~]  corpuscles;  and  in  some  there  is  a  slight  change 
in  spelling,  as  thyreoid  and  chorioid,  but  as  these  are  all  in  the 
interest  of  exactness  and  do  not  violate  morphological  princi- 
ples, they  are  employed  here. 

In  the  case  of  a  work  which,  like  the  present  one,  attempts 


VI 11 


PREFACE 


to  cover  a  large  field,  in  each  and  every  point  of  which  there 
are  opposing  views,  both  as  to  the  facts  themselves  and  to 
their  interpretation,  errors  and  misinterpretations  are  inev- 
itable, and  the  writer  craves  the  indulgence  of  those  who  have 
directed  their  special  attention  to  any  one  of  the  subjects 
touched  upon  here.  The  book  is  primarily  intended  as  an 
interpretation  of  the  work  of  the  specialists  in  anatomy, 
especially  during  the  last  half-century,  and  its  mission  will  be 
accomplished  if  it  serves  to  render  the  facts  obtained  more 
accessible  to  the  general  reader. 

DRYADS'  GREEN,  NORTHAMPTON, 
May,   1909 


CONTENTS 

PAGE 

PREFACE      v 

CHAPTER 

I.  THE  CONTINUITY  OF  LIFE i 

II.  THE  PHYLOGENESIS  OF  VERTEBRATES 26 

III.  THE  ONTOGENESIS  OF  VERTEBRATES     ....     .     .     .  48 

IV.  THE  INTEGUMENT  AND  THE  EXOSKELETON     ...  76 
V.  THE  ENDOSKELETON     .    .     .     . 122 

VI.  THE  MUSCULAR  SYSTEM 189 

• 

VII.  THE  DIGESTIVE  AND  RESPIRATORY  SYSTEMS     .      .      .  257 

VIII.  THE  VASCULAR  SYSTEM 317 

IX.  THE  URO-GENITAL  SYSTEM 365 

X.  THE  NERVOUS  SYSTEM 406 

XL  THE  SENSE-ORGANS 465 

XII.  THE  ANCESTRY  OF  THE  VERTEBRATES 506 

APPENDIX    .      .     .     . 539 


PLATES 


PLATE  I.        Diagrams  showing  Vertebrate  development ;  stages 

I  and  II.      Based  upon  a  stereogram  by  KINGSLEY.      62 

PLATE  II.      Diagrams  showing  Vertebrate  development ;  stages 

III  and  IV.    Based  upon  a  stereogram  by  KINGSLEY.      63 

PLATE  III.  Development  of  uro-genital  system  in  Amniotes 
from  stage  of  sexual  indifference  (a)  to  male  vb), 
and  to  female  (c).  In  part  after  GEGENBAUR.  .  386 

PLATE  IV.  Longitudinal  median  sections  of  Vertebrate  brains 
corresponding  to  the  first  half  of  the  series  in  Fig. 
117  in  the  text,  [(b)  and  (c)  after  EDINGER].  414 

PLATE  V.  Longitudinal  median  sections  of  Vertebrate  brains 
corresponding  to  the  second  half  of  the  series  in 
Fig.  117.  [After  EDINGER]  .  .  .  .  415 

PLATE  VI.      Diagram   of  cranial    nerves    in    Anamnia.      [After 

WIEDERSHEIM].         ......      448 

PLATE  VII.    Diagram    of  cranial    nerves    in   Amniota.       [After 

WIEDERSHEIM].         ......       449 

PLATE  VIII.  Inter-relation  of  Trigeminus,  Facialis,  Glossopharyn- 
geus,  and  Vagus,  together  with  the  sympathetic 
ganglia  in  man.  Based  upon  diagrams  by  several 
anatomists  (ARNOLD,  GRAY,  GEGENBAUR).  .  456 


"Man  still  bears  in  his  bodily  frame  the  indelible 
stamp  of  his  lowly  origin." 

CHARLES  DARWIN  :  "  Descent  of  Man  '" 
(closing  sentence) 


CHAPTER  I 
THE  CONTINUITY  OF  LIFE 

"  Ich  sage   immer  tmd  wiederhole  es,  die  Welt  konntt 
nicht  bestehen,  wenn  sie  nicht  so  einfach  ware." 

JOHANN    WOLFGANG    GOETHE,    in    Eckermann, 
Gesprache  mit  Goethe,     n  Apr.,   1827. 

OXE  of  the  grandest  generalizations  formulated  by  modern 
biological  science  is  that  of  the  continuity  of  life;  that  the 
protoplasmic  activity  within  the  body  of  each  living  being 
now  on  earth  has  continued  without  cessation  from  the  remote 
beginnings  of  life  upon  our  planet,  and  that  from  that  period 
until  the  present  no  single  organism  has  ever  arisen  save  in 
the  form  of  a  bit  of  living  protoplasm  detached  from  a  pre- 
existing portion;  that  the  eternal  flame  of  life,  once  kindled 
upon  this  earth,  has  passed  from  organism  to  organism,  and 
is  still  going  on,  existing  and  propagating,  incarnated  within 
the  myriad  animal  and  plant  forms  of  the  present  day.  Built 
up  of  carbon,  hydrogen,  oxygen,  nitrogen,  together  with 
traces  of  a  few  other  elements,  yet  of  a  complexity  of  struc- 
ture that  has  hitherto  resisted  all  attempts  at  complete 
analysis,  protoplasm  is  at  once  the  most  enduring  and  the 
most  easily  destroyed  of  substances;  its  molecules  are  con- 
stantly breaking  down  to  furnish  the  power  for  the  manifesta- 
tions of  vital  phenomena,  and  yet,  through  its  remarkable 
property  of  assimilation,  a  power  possessed  by  nothing  else 
upon  earth^itxaj)  constantly  builds  up  its  substance  anew  from 
the  surrounding  medium,  usually  in  excess  of  that  lost  by  dis- 
integration, and  possessed  of  qualities  identical  writh  those  of 
the  parent  mass.  The  continuity,  then,  is  not  one  of  ma- 
terial, but  of  qualities,  and  it  is  this  that  makes  an  organism 
the  same  from  birth  till  death.  An  acorn,  a  sapling,  an  oak 


^^:;>;  'HISTORY   OF   THE    HUMAN    BODY 

— all  are  the  same  organism,  although  the  bulk  of  the  acorn 
is  but  the  hundredth  part  of  the  sapling,  and  that  the  thou- 
sandth part  of  the  oak,  and  although  every  particle  that  con- 
stituted the  organism  in  an  early  stage  may  have  been  elim- 
inated long  before  the  next  stage  is  reached.  Upon  the  at- 
tainment of  a  certain  size-limit,  the  most  or  the  whole  of  the 
constantly  accumulating  excess  is  freed  from  the  parent  or- 
ganism, in  the  form  of  germinal  particles,  each  of  which,  still 
continuing  the  process  of  assimilation,  wrests  building  ma- 
terial from  its  surroundings,  from  other  organisms  as  well  as 
from  inorganic  substances,  and,  if  successful,  develops  into  a 
new  organism,  which  often  to  the  minutest  details  reproduces 
the  parent  from  which  it  arose. 

Through  this  power  of  assimilation  there  is  a  constant  en- 
croachment of  the  organic  upon  the  inorganic,  a  constant 
attempt  to  convert  all  available  material  into  living  substance, 
and  to  indefinitely  multiply  the  total  number  of  individual 
organisms.  This  tendency  receives  a  check,  however,  from 
two  sources :  from  the  forces  of  the  inorganic  world,  since 
each  organism  is  particularly  sensitive  to  surrounding  con- 
ditions, and,  secondly,  from  other  organisms.  It  has  been  to 
offset  these  that  all  variations  in  organisms  have  taken  place, 
changes  which  have  furnished  a  great  power  of  adaptation  to 
various  conditions  and  have  resulted  in  the  invasion  and  occu- 
pancy of  all  environments  in  which  the  conditions  do  not 
absolutely  prohibit  protoplasmic  activity. 

Thus  have  developed  all  the  plant  and  animal  forms  which 
have  ever  appeared  on  the  earth,  and  since  no  one  of  these 
can  have  arisen  spontaneously,  but  depends  for  its  develop- 
ment upon  a  bit  of  living  protoplasm  thrown  off  from  a  pre- 
viously existing  organism,  it  follows  that  all  living  beings  may 
be  traced  back  through  continuous  though  converging  lines 
of  life  to  the  first  beginning  of  all  life — the  primordial  proto- 
plasm. Difficult  as  this  may  be  to  follow  in  the  case  of  the 
more  complex  organisms,  those  which,  through  constant 
modification,  have  departed  most  widely  from  the  original 
condition,  this  continuity  of  life  is  easily  seen  in  the  one- 


THE    CONTINUITY    OF    LIFE  3 

celled  organisms,  or  Protozoa,  which  are  the  simplest  in  struc- 
ture of  all  living  things.  The  essential  body  substance  con- 
sists of  a  minute  mass  of  semi-fluid  protoplasm,  in  the  interior 
of  which  lies  a  denser  portion  which  constitutes  its  most  im- 
portant organ,  the  nucleus.  This  latter  is  the  physiological 
center  for  the  control  of  all  the  vital  functions  of  the  animal, 
and  is  undoubtedly  extremely  complex  in  structure,  even  in 
the  simplest  members  of  the  group.  In  some  protozoans  the 
protoplasm  is  enclosed  by  a  thin  but  fine  cell-membrane,  which 
preserves  for  the  animal  a  more  or  less  definite  shape ;  in  other 
cases  there  is  no  such  membrane,  and  the  protoplasm  is  free 
to  assume  an  irregular  and  constantly  changing  outline,  each 
species,  however,  still  preserving  a  certain  characteristic  range 
of  form. 

Through  the  intaking  of  other  organisms,  either  alive  or  in 
a  state  of  disintegration,  the  protoplasm  of  all  Protozoa  has 
the  power  of  adding  to  its  bulk,  through  assimilation ;  a  pro- 
cess perhaps  more  than  all  others  characteristic  of  life  and 
not  imitated  in  any  way  by  lifeless  matter.  For  this  process 
a  nucleus  is  absolutely  essential,  for  it  has  been  experimentally 
proven  that  non-nucleated  fragments  of  the  simpler  Protozoa 
are  capable  of  continuing  their  existence  for  some  time,  and 
can  even  receive  foreign  materials,  yet  have  no  power  of 
assimilation.  A  fragment  containing  a  nucleus,  on  the  other 
hand,  will  continue  to  grow  and  will^ukimately  completely 
restore  the  lost  part. 

This  process  of  growth  is  limited,  however,  not  by  any 
failure  in  the  vital  process,  but  by  the  mathematical  law  of  the 
ratio  of  surface  to  mass.*  The  intaking  of  both  food  and 
oxygen,  and  also  the  expulsion  of  all  waste  products,  take  place 
on  the  external  surface,  or,  in  the  case  of  those  covered  by  a 
cell-membrane,  over  a  restricted  portion  of  that  area,  but  on 

*  This  law  is  that  the  surfaces  of  homologous  solids  are  to  each  other 
as  the  squares,  and  their  masses  as  the  cubes,  of  their  homologous  dimen- 
sions. A  protozoan  which  has  increased  to  twice  its  normal  size,  i.  e.,  twice 
its  original  diameter,  has  increased  its  surface  four  times  and  its  mass  eight 
times.  It  has  therefore  reduced  its  proportionate  surface  by  one  half,  and 
its  supply  of  food  and  oxygen  in  the  same  degree. 


HISTORY   OF   THE    HUMAN    BODY 


account  of  the  law  just  mentioned,  the  mass  of  a  growing 
animal  increases  faster  than  its  external  surface,  and  the  time 
is  soon  reached  at  which  it  is  in  danger  both  of  starving  and 
of  suffocation.  To  offset  this,  recourse  is  had  to  a  process 
called  fission,  which  effects  at  the  same  time  a  relief  from  the 
physiological  difficulty  and  a  multiplication  of  the  individual. 
In  its  simplest  form  this  reproduction  by  fission,  as  it  is 
termed,  is  inaugurated  by  (i)  a  lengthening  of  the  nucleus; 
(2)  a  contraction  of  its  middle  portion,  producing  a  form 


FIG.  i.    Simple  fission.    Diagrams  based  on  the  infusorian  Paramcecium. 

In  all  the  figures  the  macronucleus  is  on  the  left,  the  micronucleus  on  the 
right.  The  division  of  the  micronucleus  is  effected  by  mitosis,  that  of  the  macro- 
nucleus  is  direct. 

like  an  hour-glass,  and  (3)  a  separation  of  the  two  halves, 
forming  two  independent  nuclei,  each  half  of  the  original 
size.  A  similar  subdivision  of  the  body  of  the  cell  follows, 
the  arrangement  being  such  that  each  piece  becomes  supplied 
with  one  of  the  two  nuclei,  and  is  capable  of  beginning  an 
independent  existence.  In  certain  other  cases  the  proceeding 
is  more  complicated.  The  organism  surrounds  itself  with  a 
shell  or  cyst,  secreted  by  the  protoplasm,  and  after  a  quiescent 
period,  breaks  up,  not  into  two,  but  a  larger  number,  usually 
four,  eight  or  sixteen,  which  become  released  by  the  bursting 
of  the  cyst  and  swim  out  into  the  water,  each  in  its  turn  to 
assimilate  foreign  matter  until  of  the  size  for  another  encyst- 
ment. 

It  is  but  natural  to  refer  to  the  undivided  organism  as  the 


THE    CONTINUITY   OF    LIFE  5 

"  parent,"  and  to  the  resultant  organisms,  whether  two  or 
one,  as  the  "  offspring,"  yet  it  is  here  plain  that  we  do  not 
have  to  do  with  either  parent  or  children  in  the  usual  sense. 
The  "  parent,"  as  such,  ceases  to  exist  the  moment  it  becomes 
divided;  yet  no  death  has  ensued,  for  there  is  no  dead  body. 
The  vital  activity  of  protoplasm  has  been  perpetuated,  without 
an  interruption,  from  the  undivided  mass  to  each  piece  result- 
ing from  the  fission,  or  in  other  words,  the  life  is  continuous. 
In  a  restricted  sense,  then,  a  protozoan  is  immortal:  its 


FIG.  2.     Multiple  fission  as  shown  by  the  parasite  of  malaria,    Ha-ma- 
morba   malaria.     [After  Ross  and  FIELDING-OULD.] 

The    enclosing    outline    represents    a    human    blood    corpuscle,    within    which    the 
transformation   takes   place. 

(a)  Young  amoeboid  stage  formed  from  a  sporozoid.  (b)  Older  amoeboid  stage, 
showing  growth,  (c)  Beginning  of  multiple  fission,  .(d)  Division  of  the  mass 
into  eight  sporozoids.  At  this  stage  the  sporozoids  become  liberated  through  the 
distintegration  of  the  remains  of  the  corpuscle,  and  invade  the  plasma.  From  this 
they  enter  new  corpuscles,  and  assume  the  amoeboid  form  as  at  a,  thus  completing 
the  cycle. 

vital  activities  have  been  continuous,  without  interruption  from 
the  beginning  of  life  upon  the  planet.  It  is  not  meant,  of 
course,  that  a  protozoan  is  indestructible,  for  countless  num- 
bers of  them  are  continually  succumbing  to  mechanical  or 
chemical  injury;  but  each  accident  of  this  sort  extinguishes  a 
life  which  has  existed  without  cessation  from  the  first  life  of 
all.  The  actual  material  particles  are  constantly  changing, 
even  while  a  protozoan  is  retaining  its  identity  as  an  indi- 
vidual, yet  that  which  is  continuous  from  moment  to  moment 
in  such  an  individual,  is  equally  so  during  and  after  each 
fission,  and  is  perpetuated  without  interruption,  in  each  piece, 
so  long  as  it  does  not  meet  with  conditions  which  destroy  it. 
Aside  from  the  phenomena  of  reproduction  by  fission,  there 
is  another  procedure  which  has  been  observed  in  many  forms 


6  HISTORY   OF   THE   HUMAN   BODY 

of  Protozoa,  and  while  in  the  present  state  of  knowledge  it 
cannot  be  asserted  that  it  is  a  universal  procedure,  existing  in 
all  species,  it  is  very  likely  that  this  or  a  similar  process  is  oc- 


FIG.  3.  Conjugation.  Diagrams  based  on  the  infusorian  Paramacium. 
Here  the  two  gametes  are  of  the  same  size  and  the  fusion  is  temporary 
with  similar  results  in  the  case  of  each. 

(a)  The  two  micronuclei  are  forming  mitotic  figures  preparatory  to  division, 
(b)  The  two  micronuclei  have  elongated;  the  macronuclei  are  disintegrating.  (c) 
One-half  of  each  micronucleus  passes  into  the  other  individual  through  the 
mouth,  (d)  Fusion  occurs  in  each  individual  between  the  half  nucleus 
that  originally  belonged  to  it  and  the  half  nucleus  that  has  come  from 
the  other.  This  forms  a  fusion-nucleus,  (e)  The  fusion-nuclei  form  mitotic  figures 
preparatory  to  division.  At  about  this  time  the  two  individuals  separate.  (f),  (g), 
(h)  The  fusion-nucleus  divides  three  times  in  succession,  eventually  forming  eight 
nuclei.  (i)  Four  of  the  eight  nuclei  enlarge  and  form  macronuclei,  and  four  re- 
main small  and  become  micronuclei.  These  become  associated  in  pairs,  one  micro- 
and  one  macro-nucleus,  and  are  distributed  to  four  individuals  that  result  from 
two  successive  divisions.  Each  of  these,  evidently  as  the  result  of  the  conjugation, 
has  a  renewed  power  of  fission,  and  multiplication  continues  in  this  way  [cf.  Fig.  i] 
until  the  power  becomes  diminished,  when  it  is  renewed  by  a  new  conjugation 
[cf.  Fig.  5  (a)]. 

casionally  undergone  in  all  cases.  This  is  the  process  of  con- 
jugation [Fig.  3]  which,  in  the  cases  best  studied,  seems  to 
bear  a  definite  relation  to  the  process  of  reproduction  by 
fission.  In  these  cases  the  number  of  fissions  which  can  occur 
in  succession  appears  to  be  limited,  for  after  a  series  of  these 


THE   CONTINUITY   OF   LIFE  7 

it  seems  that  the  reproductive  force  becomes  lessened,  causing 
longer  pauses  between  successive  fissions,  and  ultimately  the 
death  of  the  organisms.  It  is  at  this  time,  when  the  fissions 
are  farther  between  and  carried  on  with  less  activity,  that 
conjugation  appears.  This  consists  typically  of  the  temporary 
fusion  of  two  individuals,  during  which  there  is  a  mutual  inter- 
change of  certain  of  the  elements  of  the  nuclei.  When  this 
is  accomplished  the  two  individuals,  or  gametes,  as  they  are 
here  termed,  separate,  and  begin  anew  a  fresh  series  of  fissions 
as  at  first. 

The  purpose  of  the  process  thus  seems  to  be  something  like 
a  rejuvenescence,  by  means  of  which  the  reproductive  activity 
may  be  renewed;  yet,  that  the  action  is  chemical  rather  than 
physiological  is  suggested  by  experiments  in  which  a  similar 
increase  of  activity,  taking  the  place  of  conjugation,  may  be 
induced  by  the  addition  of  food-substances  like  beef  broth  to 
the  water  containing  the  species  under  investigation. 

In  many  cases  the  process  of  conjugation  is  rendered  more 
complicated  by  the  introduction  of  two  sorts  of  individuals, 
macro-  and  micro-gametes,  which  are  evidently  produced  for 
this  especial  purpose  by  a  variation  in  the  usual  course  of  the 
fission  process.  In  this  case  the  two  usually  unite  perma- 
nently and  form  a  zygote,  which  becomes  thus  endowed  with 
special  reproductive  activity.  [Fig.  4.] 

In  multicellular  organisms  the  matter  becomes  still  more 
complicated,  but  is  essentially  the  same  so  far  as  concerns  pro- 
toplasmic continuity.  Here  only  certain  cells,  which  are  called 
germ-cells ,  act  as  gametes  and  conjugate,  producing  the  new 
organisms  by  their  repeated  divisions,  while  the  remainder, 
often  vastly  preponderating  over  the  former  in  actual  bulk, 
build  up  a  body  or  so  ma,  which  forms  a  shelter  and  protection 
for  the  germ  cells.  Somata  possess  a  high  degree  of  adapt- 
ability to  external  conditions,  and  become  modified  to  fit  them, 
so  that  in  this  way  they  and  the  germ-cells  contained  within 
them  may  come  to  be  developed  in  places  and  under  circum- 
stances where  otherwise  they  could  not  possibly  exist. 

In  this  way  all  animal  and  plant  forms  have  been  produced, 


8 


HISTORY    OF    THE    HUMAN    BODY 


FIG.  4.  Carchesium,  a  sessile  protozoan  colony,  showing  conjugation. 
[Diagram  in  part  after  BUTSCHLI  and  SCHEWIAKOFF.] 

a  a  Macrozooids,  which,  by  their  division  produce  either  b,  other  macrozooids,  or 
c  microzooids,  which  eventually  become  free,  d  Free-swimming  microzooids,  per- 
haps from  another  colony,  d  microzooid  (here  a  microgamete)  in  conjugation  with 
a  macrozooid  (macrogamete).  In  each  of  the  above  individuals  may  be  seen  a 
vermiform  marcronucleus  and  a  spherical  micronucleus.  e  Detail  of  conjugation. 
The  macronucleus  of  each  component  is  shown  broken  into  fragments  previous  to  dis- 
solution; the  two  micronuclei  are  dividing  mitotically  into  two  halves,  one-half  of  each 
destined  to  pass  into  the  other  component.  The  micro-  and  macro-gametes  are 
designated,  respectively,  as  male  and  female.  [Subsequent  stages  similar  to  those 
shown  in  Fig.  3.] 


THE    CONTINUITY    OF    LIFE  9 

each  being  but  the  temporary  dress  of  a  proliferating  mass  of 
protoplasm;  a  detached  mass  of  tissue,  which  feeds,  breathes, 
and  often  moves  and  perceives,  for  the  better  support  and  pro- 
tection of  the  continuous  living  protoplasm.  The  soma  is 
mortal,  and  after  a  longer  or  shorter  period  loses  its  vitality 
and  goes  to  dissolution;  the  germ,  in  the  restricted  sense  of 
being  coeval  with  life  upon  the  earth,  is  immortal;  and  yet,  in 
spite  of  the  far  greater  value  of  the  latter,  the  two  are  very 
closely  associated.  As  the  soma  becomes  modified,  the  germ 
becomes  equally  so,  since  each  germ,  as  it  develops,  repro- 


A     A 


^  _ 

A  A  *    A    A   A,J_A    .A.  A          A 

A  '  "  A   A  ~  ATTuA       AMAj 

•  •      •   •    s  t     yrfrfrtiTT"""8'  «^sffff*«***« 


FIG.  5.  Diagrams  illustrating  the  life  cycle  in  unicellular  and  multicel- 
lular  organisms. 

The  round  dots  represent  cells.  In  (b)  and  (c)  the  germ  cells  are  gray,  the 
somatic  cells  black.  In  all  cases  the  destruction  of  a  cell  is  indicated  by  a  heavy 
black  bar  placed  beneath  it. 

(a)  Protozoan    type,    with    equivalent    gametes.      The    series   begins    with    a    con- 
jugation,  after  which  the  gametes  separate  and  a  series  of  simple  fissions   follows  in 
the  case  of  each  gamete.     After  several  generations  of  these,   in  which  many  of  the 
individuals  produced   are   destroyed,   conjugation   again  appears,   completing  the  cycle. 

(b)  Life  cycle  in  a  male  Metazoan.     The  cycle  begins  with  a  conjugation  between 
a   macro-    and   a   micro-gamete    (ovum    and    spermatozoon),    after   which    there    follows 
a   series    of   simple   fissions,    which    differ    from    those    of    (a)    in   the   perpetual    union 
of    the    components    thus    formed,    represented    here    by    connecting    lines.      There    is 
thus  built  up  an  interdependent  cell-colony,   the   soma,   shown  in  the  fifth  row   from 
the  top.     Certain  of  the  somatic  cells  become  microgametes   (^spermatozoa),  destined 
for    conjugation,    and    capable    of    independent    existence    when    separated    from    the 
rest.      The   remaining   somatic   cells   perish   simultaneously. 

(c)  Same  as  (b),  but  th'e  gametes  produced  are  macrogametes  (  =  ova),  and  the 
soma  is  consequently  female.  The  conjugation  of  these  cells  with  microgametes  is 
shown  in  the  lower  row,  thus  completing  the  life  cycle.  The  bars  placed  beneath 
the  completed  germ  cells  in  (b)  and  (c)  suggest  the  probable  proportion  of  accidental 
destruction. 

duces  a  soma  almost  identical  with  that  from  which  it  came; 
a  result  which  can  be  explained  only  by  supposing  that  each 
germ  contains  a  controlling  mechanism,  directing  and  de- 
termining the  development  of  every  individual  part  in  the 
future  soma. 


io  HISTORY   OF   THE    HUMAN    BODY 

The  differences  between  unicellular  and  multicellular  organ- 
isms in  these  respects  may  be  graphically  expressed  in  the 
accompanying  diagrams.  [Fig.  5.] 

In  the  first  of  these  (a),  which  represents  the  condition  in 
the  simpler  unicellular  animals,  a  cycle  of  cell  generations 
begins  with  a  conjugation,  a  procedure  during  which  a  part 
of  the  nuclear  material  of  the  two  conjugating  individuals  is 
mutually  exchanged,  the  result  seeming  to  be  an  increased 
activity  of  division  for  some  time.  The  resulting  cell  genera- 
tions are  followed  in  the  diagram  in  the  case  of  but  one  of 
the  two  conjugating  individuals,  that  of  the  other  being  sim- 
ilar. Several  generations  are  indicated,  as  also  the  chance 
mortality  of  individuals,  the  result  of  this  last  being  to  keep 
the  total  number  of  individuals  in  each  generation  approxi- 
mately the  same  in  spite  of  the  geometrical  ratio  in  which  the 
individual  cells  tend  to  increase. 

The  two  other  diagrams  (b)  and  (c)  represent  a  similar 
cycle  of  cell  generations  in  two  multicellular  organisms,  male 
and  female,  respectively.  In  these,  the  cycle  begins  with  the 
union  of  a  male  and  female  germ-cell,  that  is,  a  permanent 
conjugation  between  a  micro-  and  a  macro-gamete,  forming 
a  fertilized  ovum.  Because  of  the  cellular  differentiation  due 
to  a  necessary  adaptation,  the  male  cell  is  small  and  active  and 
equipped  with  a  locomotive  organ  in  the  form  of  a  vibratile 
flagellum,  while  the  female  cell  is  more  or  less  immobile  and 
furnished  with  a  large  amount  of  yolk,  the  food  supply  for 
the  embryo  during  its  early  development,  when  it  cannot  ob- 
tain its  own  nourishment.  After  the  conjugation  there 
ensues  a  series  of  cell  generations,  as  in  the  other  case,  with 
the  essential  difference  that  here  they  remain  in  organic  con- 
tinuity with  one  another  and  form,  not  independent  indi- 
viduals, but  the  component  parts  of  a  multicellular  organism. 
The  number  of  such  generations  is  often  very  great,  certainly 
much  greater  than  here  represented,  and  the  cells  early  begin 
a  differentiation  of  form  and  function  which  leads  eventually 
to  the  formation  of  all  the  tissues  necessary  to  build  up  the 
adult  body  or  soma.  Among  those  early  cells  are  the 


THE    CONTINUITY   OF   LIFE  n 

primordial  germ-cells,  differently  marked  in  the  diagrams, 
which  seem  to  retain  the  general  qualities  of  the  first  egg-cell 
and  to  resist  the  tendency  to  specialization  seen  in  the  others. 
From  these  the  final  germ-cells  develop,  small  and  mobile  in 
the  case  of  the  male,  large  and  provided  with  yolk  in  the  case 
of  the  female.  These,  liberating  themselves  from  the  soma, 
unite  in  pairs  to  form  another  cycle  like  the  first,  while  all  the 
generations  of  the  somatic  cells  are  sooner  or  later  brought 
to  an  end  simultaneously,  the  death  of  the  individual. 

This  organic  connection  between  the  cells,  which  constitutes 
the  essential  difference  between  unicellular  and  multicellular 
organisms,  has  its  advantages  as  well  as  its  disadvantages. 
The  chief  among  the  first  is  the  great  power  of  differentiation 
among  individual  cells  or  cell-groups,  with  the  resultant  di- 
vision of  labor;  a  great  disadvantage  lies  in  the  fact  that 
through  this  very  specialization  of  function,  any  vital  accident 
occurring  in  one  part  drags  down  to  death  all  the  other  cells 
of  the  organism.  The  germ-cells  alone  are  the  immortal 
parts,  the  continuous  principle  which  survives  the  destruction 
of  the  soma,  and  each  contains  within  itself,  expressed  in  the 
form  of  an  ultra-complex  mechanism,  the  ability  to  reproduce 
in  its  cell  descendants  every  detail  of  the  soma  from  which 
it  originated. 

In  this  is  seen  the  primary  value  of  the  soma,  which  be- 
comes clear  when  taken  in  connection  with  the  struggle  on  the 
part  of  nature  to  develop  as  much  protoplasm  as  possible. 
The  soma  is  a  mass  of  protective  cells,  capable  of  a  high 
degree  of  specialization,  and  thus  able  to  adapt  itself  in  accord- 
ance with  the  needs  of  every  environment  in  which  it  is 
possible  for  organic  beings  to  exist.  Even  its  death  is  an 
adaptation,  for  by  this  means  new  and  perfect  somata  are  con- 
stantly taking  the  place  of  those  whose  usefulness  as  guardians 
of  the  germ-cells  has  become  impaired  by  the  inevitable  injury 
to  which  organisms  are  constantly  exposed.  Life  is  con- 
tinuous in  the  germ-cells  from  generation  to  generation  and 
has  been  carried  into  all  environments  and  protected  and 
multiplied  through  a  constant  succession  of  perishable  somata. 


12  HISTORY   OF   THE    HUMAN    BODY 

The  adaptations  of  the  soma  are  extremely  gradual,  and 
thus,  if  all  forms  that  have  ever  existed  could  be  arranged  in 
order,  they  would  form  a  continuous  series,  not  in  the  form  of 
a  straight  line,  but  in  that  of  a  profusely  branching  tree,  since 
from  one  parent  form  two  or  more  varieties  are  constantly 
arising,  capable  of  inhabiting  a  slightly  different  environment, 
and,  if  successful,  continuing  along  separate  lines  of  develop- 
ment. As  a  matter  of  fact,  however,  the  fauna  and  flora  of 
the  world  at  present  represent,  for  the  most  part,  but  isolated 
units  in  the  great  system,  and  while  a  careful  study  of  the 
structure  of  every  known  form  has  led  to  the  restoration  of 
many  portions  of  this  tree,  there  are  in  other  places  great  gaps 
filled  thus  far  only  by  inferences,  and  therefore  matters  of 
continual  controversy. 

v  This  continuity  of  all  life  and  the  recognition  of  animals 
and  plants  as  no  more  than  the  countless  adaptive  forms  of 
the  plastic  soma,  enable  the  zoologist  to  trace  out  with  con- 
siderable accuracy  the  history  of  those  series  of  which  the 
records  are  the  best  preserved,  a  history  which,  while  lying  in 
the  past,  is  represented  in  the  present  by  forms  which  arose  in 
earlier  periods,  the  complete  adaptation  of  which  has  allowed 
them  to  successfully  struggle  with  their  competitors  and  thus 
to  survive  with  but  little  change  to  the  present  day.  It  is  in 
this  sense,  then,  that  there  can  be  a  history  of  the  human  body, 
the  history  of  the  struggles  and  successes  and"  failures  of  our 
remote  ancestors,  as  they  successively  encountered  the  various 
environments  wherein  this  history  has  been  enacted.  The 
ocean,  the  marsh,  the  prairie,  the  forest,  each  has  formed  the 
complex  stage-setting  of  an  historic  period  and  has  contributed 
to  the  formation  of  the  human  soma.  Man's  body  was,  like  all 
others,  not  made  new,  but  adapted,  and  this  not  once,  but 
repeatedly.  Old  organs  have  been  readapted  to  new  uses  or 
are  retained  as  merely  functionless  rudiments,  new  organs  have 
arisen  through  the  change  of  function  of  some  preexisting 
part,  the  body  has  in  all  its  details  been  molded  and  shaped 
with  each  new  change  to  the  end  of  producing  the  highest 
degree  of  physiological  efficiency,  and  this  always  with  sole 


THE    CONTINUITY    OF   LIFE  13 

reference  to  the  problem  in  hand  and  with  no  regard  to  the 
future  inconveniences  which  may  arise  from  a  certain  form 
or  arrangement. 

To  learn  this  history  we  must  turn  to  the  comparative 
anatomy  of  vertebrates.  Some  of  them  are  still  so  similar 
to  the  early  stages  of  our  own  development  that  we  may 
almost  look  upon  them  as  our  former  selves;  others  represent 
development  along  other  lines  to  which  their  environment  and 
its  necessities  have  brought  them,  and  they  show  us  what  we 
might  have  been,  had  chance  led  us  in  their  direction. 

The  first  period  of  vertebrate  history  was  an  aquatic  one, 
in  which  the  environment  was  represented,  not  merely  by  the 
water,  which  developed  a  certain  kind  of  respiration,  and  al- 
lowed a  style  of  locomotive  organs  inadmissible  on  land,  but 
by  the  vast  hordes  of  carnivorous  enemies  generated  in  the 
depths  of  the  ocean;  yet,  through  these  struggles  was  gained 
an  exoskeletal  armor  with  which  to  ward  off  the  attacks  of 
the  powerful  molluscs  and  crustaceans  of  the  Silurian  seas; 
and  of  the  armor  plates  thus  obtained  the  relics  are  still  re- 
tained in  the  cranial  region,  forming  the  dermal  bones  of  the 
skull  (f rentals,  parietals,  squamosals,  etc.). 

Profound  changes  became  necessary  when  our  ancestors 
left  the  ocean  and  sought  refuge  in  the  marshes  and  upon 
land;  changes  not  merely  in  the  mode  of  respiration,  but  in 
the  entire  skeletal  and  muscular  system,  owing  to  the  great 
difference  in  specific  gravity  between  water  and  air.  Differ- 
ences in  food  caused  modifications  in  the  digestive  system, 
and  all  surfaces  exposed  to  the  air  developed  glands  in  pro- 
fusion to  resist  the  drying  effect  of  sun  and  wind.  During 
this  period  were  acquired  pentadactylous  extremities,  lungs 
and  larynx,  and  the  salivary  and  lacrimal  glands.  The  or- 
ganism became  modified  in  countless  ways,  as  the  attempts  to 
inhabit  dry  lands,  apart  from  the  marshes,  ushered  in  the  next 
great  period,  that  of  the  rocks  and  plains. 

Here  began  a  complete  aerial  respiration,  the  development 
of  the  permanent  kidneys,  which  replaced  the  Wolffian  bodies 
of  amphibians,  and  the  formation  of  a  cornified  epidermis,  with 


14  HISTORY    OF   THE    HUMAN    BODY 

its  proliferations  in  the  form  of  scales,  horns  and  claws.  The 
great  increase  in  the  size  and  strength  of  the  limbs,  begun  in 
the  previous  period,  reached  here  a  high  degree  of  perfection, 
and  towards  the  end  of  this  period  vertebrates  were  for  the 
first  time  enabled  by  the  help  of  these  to  lift  their  bodies  com- 
pletely from  the  ground  and  exchange  the  crawling  move- 
ments for  a  definite  walk. 

But  the  most  important  of  all  the  changes  produced  by  a 
land  environment  has  been  the  rapid  increase  in  the  size  and 
efficiency  of^  the  central  nervous  system,  which  became  de- 
veloped in  part  through  the  need  of  controlling  the  larger  limb 
muscles,  and  in  part  in  response  to  the  far  more  varied 
environment  afforded  by  the  land  surfaces  and  the  consequent 
necessity  of  recording  a  larger  number  of  sensory  impressions. 
By  a  curious  and  indirect  method  this  development,  especially 
that  of  the  perceptive  centers  of  the  brain,  has  been  still  more 
encouraged  in  a  certain  group  of  rather  generalized  mammals 
through  the  occupation  of  an  arboreal  environment.  The 
direct  result  of  this  was,  that  in  these  animals,  which  were,  in 
the  main,  large  enough  to  grasp  the  boughs  in  climbing,  a 
prehensile  paw  with  an  opposable  first  digit  was  developed  on 
both  anterior  and  posterior  limbs,  and  this  new  tool,  especially 
the  anterior  set,  which  became  hands,  from  now  on  allowed 
the  animals  to  grasp  all  sorts  of  objects,  and  expose  them  to 
a  more  careful  scrutiny,  thus  causing  a  continually  greater 
development  of  the  recording  centers  of  the  brain. 

As  this  arboreal  environment  has  been  the  latest  in  the 
line  of  human  history  previous  to  the  assumption  of  a  strictly 
terrestrial  life,  there  are  still  in  man's  body  more  evidences 
of  this  than  of  the  earlier  stages,  but  these,  because  they  are 
the  latest,  are  also  the  most  superficial,  and  consist  of  such 
characters  as  the  flattened  nails,  the  pectoral  position  of  the 
mammae,  and  the  opposable  thumbs. 

The  latest  change  of  all,  the  assumption  of  an  erect  position 
and  the  emancipation  of  the  anterior  limbs  from  all  locomotive 
functions,  has  necessitated  a  few  modifications,  especially 
changes  in  the  pelvic  girdle  and  in  the  relative  size  and 


THE    CONTINUITY    OF   LIFE  15 

strength  of  the  muscles  of  the  legs,  but  has  effected  little  in 
the  way  of  actual  change  of  structure,  so  that  anatomically 
man  still  stands  very  near  his  arboreal  kinsmen  that  represent 
the  immediate  past  in  the  history  of  human  development. 

This  study  of  the  succession  of  forms  upon  the  earth  is 
termed  race  history  or  phylogenesis,  and  forms  one  of  the  two 
sources  from  which  the  past  history  of  animal  development 
may  be  obtained.  The  other  is  the  sequence  of  stages  re- 
corded during  the  embryonic  development  of  each  individual, 
and  is  termed  the  developmental  history  or  ontogenesis.  By 
what  is  at  once  the  most  natural  and  the  most  mysterious  law 
of  nature  each  individual  animal  inherits,  not  only  the  struc- 
ture of  its  immediate  parents,  the  attainment  of  which  means 
the  end  of  its  development,  but  also  that  of  its  entire  line  of 
ancestors,  which  appear  in  approximately  the  natural  order 
of  succession  and  constitute  the  stages  of  its  ontogenetic  de- 
velopment. 

As  a  result  of  this  it  follows  that  the  two  records,  phylo- 
genetic  and  ontogenetic,  run  closely  parallel,  and  each  serves 
in  many  places  to  bridge  a  gap  or  explain  an  obscure  period 
in  the  other.  This  parallelism  of  the  two  records  lies  at  the 
basis  of  all  morphological  speculation,  and  forms  what  is 
often  termed  the  law  of  biogenesis*  It  must  not  be  expected, 
however,  that  the  correspondence  in  the  two  records  is  com- 
plete, since  numerous  disturbing  causes  must  be  taken  into 
consideration  which  tend  to  modify  each  record  quite  inde- 
pendently of  the  other.  In  the  race  history  there  are  many 
gaps  caused  by  extinction,  and  the  forms  that  have  come 
down  to  us  from  earlier  periods  have  become  much  changed 
from  their  former  condition  and  represent  their  ancestors  in 
a  qualified  sense  only;  while  in  the  individual  development 
there  are  many  characters  that  are  in  no  sense  historic,  and 
have  to  do  with  such  immediate  environmental  problems  as 
nutrition  or  protection.  These  latter  characteristics,  which 

*  The  "  Biogenetisches  Grundgesetz "  of  Haeckel ;  formulated  by  him 
as  follows:  "Die  Ontogenie  (Keimesgeschichte)  ist  eine  kurse  IVieder- 
hohmg  der  Phylogenie  (Stammesgeschichte)." 


16  HISTORY   OF   THE    HUMAN    BODY 

are  called  c&nogerietic,  or  modern,  are  clearly  of  no  importance 
in  such  inquiries  as  the  present,  and  must  be  carefully  distin- 
guished from  those  that  are  palingenetic,  that  is,  actual  repe- 
titions of  past  history. 

It  is  essential,  then,  in  order  to  interpret  correctly  the  two 
records,  phylogenetic  and  ontogenetic,  and  from  them  to  re- 
produce the  past  history  of  our  race,  with  its  solutions  of  the 
details  of  man's  structure,  that  the  nature  of 'each  form  of 
record  be  thoroughly  understood.  The  phylogenetic  or  race- 
history  is  the  plainer  and  more  direct  of  the  two,  and  presents 
fewer  technical  difficulties  to  the  student,  but  it  contains  at 
present  extensive  gaps,  not  yet  filled  in  by  the  discovery  of 
fossil  remains;  the  manuscript  is  plain  and  clear,  but  has  suf- 
fered much  from  the  ravages  of  time  and  is  fragmentary  at 
best :  the  ontogenetic,  on  the  other  hand,  presents  a  more  con- 
tinuous story,  but  the  difficulties  in  the  way  of  investigation 
are  very  great;  here  the  manuscript  is  written  in  a  micro- 
scopic hand,  and  is,  moreover,  a  palimpsest,  scribbled  over  with 
extraneous  material,  added  at  late  dates  and  connected  with 
the  exigencies  of  development. 

The  characteristics  of  the  phylogenetic  record  may  be  made 
clear  by  the  aid  of  the  accompanying  diagram  [Fig.  6],  which 
represents  a  purely  hypothetical  case,  and  the  conditions  in- 
volved may  be  presented  in  the  form  of  laws,  as  follows : 

I.  Development  has  not  been  in  a  single  direction,  but  in 
many,  since  the  constant  rivalry  between  allied  forms  causes 
them  to  continually  push  their  vvay  into  neiv  environments, 
the  gradual  adaptation  to  which  causes  a  greater  and  greater 
divergence  between  the  descendants  of  those  that  entered  the 
new  environment  and  those  that  remained  in  the  old. 

To  illustrate  this  by  the  diagram,  suppose  29  to  represent  a 
terrestrial  carnivorous  animal,  living  on  the  border  of  the 
ocean  and  preying  upon  the  forms  of  life  found  upon  the 
shore,  or  within  shallow  water.  Pressed  by  the  struggle  for 
existence,  in  this  instance  represented  by  the  scarcity  of  this 
sort  of  food,  certain  individuals  venture  farther  out  into 
deeper  water  and  attempt  to  capture  fish.  Thus  begins  the 


THE    CONTINUITY   OF   LIFE  17 

establishment  of  a  group  which  becomes  more  and  more 
aquatic,  as  represented  by  the  divergent  line  leading  to  34, 
until  finally  a  completely  aquatic  fish-eating  animal  or  group 
of  animals  is  the  result,  the  form  at  the  end  of  the  line,  34, 
representing  the  highest  point  of  specialization  attained. 

The  remaining  descendants  of  29,  continuing  to  live  in  pre- 
cisely the  same  habitat  as  their  ancestors,  as  is  here  indicated 


15 


FIG.  6.  Hypothetical  tree  illustrating  the  interrelations  of  organisms. 

Extinct  forms  are  represented  by  open  circles,  living  forms  by  solid  black  ones. 
The  same  number  distinguished  by  exponent  letters  signifies  a  close  relationship. 
The  dotted  areas  suggest  some  special  environment,  the  inhabitants  of  which  show 
"  adaptive  resemblance  "  although  representing  several  unrelated  lines. 

by  the  continuance  of  the  line  29-35,  m  the  same  direction  as 
28-29,  either  remain  exactly  as  their  ancestors,  or  probably 
become  more  highly  specialized  in  the  same  direction.  Con- 


i8  HISTORY   OF   THE    HUMAN    BODY 

tinual  divergencies  on  the  part  of  animals,  as  they  seek  new 
environments  in  this  way,  produce  the  numerous  divergent 
branches,  the  relative  time  of  the  divergence  being  expressed 
by  the  position  of  the  intersection  and  the  amount  of  the  mod- 
ification by  the  length  of  the  line. 

II.  Although  animal  forms  are  not  related  to  one  another 
as  members  of  a  single  linear  series,  they  yet  form  a  con- 
tinuum, and  any  two  living  forms,  however  great  the  struc- 
tural difference  between  them,  are  connected  to  one  another 
by  a  continuous  chain  of  animals,  a  connection  which  will 
become  apparent  by  tracing  the  lines  backwards  along  the" 
ancestral  course  of  each  until  they  meet  at  their  earliest  com- 
mon ancestor. 

Thus,  in  tracing  the  relationships  of  9  and  14,  neither  form 
is  ancestral  to  the  other,  but  both  arose  from  the  common 
ancestor  7,  back  of  which  their  history  is  identical.  As  the 
ancestral  forms  are  now  wholly  extinct,  they  are  no  longer 
available  for  study  save  when  found  in  the  fossil  state,  but 
their  place  may  often  be  supplied  by  modern  forms  which  are 
but  little  modified  from  the  condition  of  the  actual  ancestors. 
Thus  the  recent  forms  8a  and  7a  are  almost  as  useful  in  re- 
producing this  part  of  the  phylogenetic  history  as  8  and  7 
would  be,  and  through  them  the  inter-relationship  of  9  and 
14  may  be  readily  traced.  This  may  be  stated  as  a  third  law : 

III.  Although  the  actual  ancestral  forms  lying  at  the  fork- 
ing of  the  branches  no  longer  exist  and  have  seldom  been 
found  in  a  fossil  state,  many  clews  of  their  structure  may  be 
obtained  by  the  study  of  those  of  their  descendants  which 
have  retained  most  completely  the  ancestral  environment,  and 
which  have,  therefore,  kept  many  or  most  of  the  ancestral 
characteristics. 

Thus  in  studying  the  relationships  and  comparing  the  struc- 
ture of  two  such  divergent  forms  as  32  and  34,  the  living  form 
300  would  be  of  the  greatest  assistance,  as  it  would  enable 
the  investigator  to  see  what  was  the  common  structural 
heritage  from  which,  through  two  lines  of  modification,  the 
two  forms  in  question  have  developed. 

IV.  Among  the  fossil  remains  of  extinct  forms  which  geo- 


THE   CONTINUITY   OF   LIFE  19 

logical  investigation  has  unearthed,  many  forms  have  been 
found  which  are  the  actual  ancestors  of  groups  now  distinct, 
and  they  have  thus  been  of  the  greatest  value  in  tracing  out 
phylo genetic  relationships.  Others,  however,  represent  a 
series  of  forms  which  developed,  culminated  and  became  ex- 
tinct before  modern  times,  thus  presenting  a  group  of  great 
value  to  the  student,  but  having  no  bearing  upon  the  present 
discussion. 

Perhaps  the  most  famous  of  the  ancestral  forms  found  in 
a  fossil  state  is  the  Archccopteryx,  a  definite  transition  be- 
tween reptiles  and  birds.  Of  this,  two  specimens  were  discov- 
ered in  the  lithographic  slate  quarry  at  Solenhofen,  Germany. 
Others,  of  almost  equal  importance,  have  assisted  greatly  in 
suggesting  the  relationship  between  amphibians  and  reptiles, 
and  have  furnished  clews  to  the  proper  arrangement  of  the 
orders  of  living  mammals.  As  illustrations  of  large  groups 
of  animals  whose  history  lies  wholly  in  the  past  may  be  men- 
tioned the  trilobites,  a  group  of  crustacean-like  articulates, 
which  became  wholly  extinct  at  the  end  of  the  Palaeozoic 
Age,  and  the  ammonites,  a  group  of  cephalopod  molluscs. 

V.  The  relative  amount  of  structural  difference  between 
any  two  divergent  forms  is  proportionate  to  the  amount  of 
contrast  between  their  environments,  and  not  necessarily  to 
the  amount  of  time  that  has  elapsed  since  their  divergence 
from  the  common  ancestor. 

That  time  has  in  itself  no  power  to  modify  an  animal 
species  is  shown  by  the  slight  differences  that  exist  in  some 
cases  between  certain  living  forms  and  their  fossil  allies. 
Perhaps  the  most  conspicuous  example  of  this  is  the  brachio- 
pod,  Lingula,  a  worm  enclosed  in  a  bivalve  shell.  This  form 
has  existed  from  the  earliest  Silurian  times  to  the  present  day, 
and  yet  there  are  hardly  sufficient  differences  between  the 
earliest  fossil  Lingula  and  those  now  alive  to  allow  them  to 
be  treated  as  distinct  species.  As  a  rule,  however,  successive 
geological  periods  show  almost  a  complete  change  in  their 
fauna  and  flora,  and  most  of  the  modern  forms  are  quite 
recent  in  origin. 

The  persistence  of  ancestral  types  in  a  slightly  modified 


20  HISTORY    OF   THE    HUMAN    BODY 

condition  is  indicated  in  the  diagram  by  such  forms  as  30  or 
300  where  the  shortness  of  the  line  connecting  the  living  form 
with  its  ancestor  indicates  but  little  change  from  the  earlier 
condition. 

VI.  As  a  given  environment  tends  to  exert  a  similar  influ- 
ence upon  all  of  its  occupants,  members  of  quite  distantly 
related  groups  which  become  associated  in  the  same  environ- 
ment often  become  so  similarly  influenced  as  to  bear,  super- 
ficially, at  least,  a  great  resemblance  to  one  another.  This  is 
called  " .analogical  resemblance"  and  has  been  productive  of 
many  mistakes  in  the  attempt  to  clear  up  phylogenetic  rela- 
tionships. 

Many  striking  examples  of  this  are  found  among  verte- 
brates. Thus  a  pelagic  environment,  as  seen  among  the 
extinct  ichthyosaurs  and  the  modern  Cetacea,  has  changed 
the  fore-limbs  into  fin-like  paddles,  reduced  the  hind-limbs  to 
functionless  rudiments,  shortened  the  neck,  and  given  head 
and  body  a  piscine  form;  limbless,  attenuated  forms  occur 
among  fishes,  amphibians  and  several  groups  of  reptiles  other 
than  snakes ;  and  a  grazing  habit  produced  in  the  herbivorous 
reptilian  group  of  the  dinosaurs  a  close  resemblance  to  the 
large  ungulate  mammals  of  a  later  day. 

This  law  is  illustrated  in  the  diagram  by  the  forms  included 
by  the  dotted  line,  which  represents  a  given  environment,  in- 
vaded by  members  of  several  groups.  Here  the  descendants, 
not  only  of  related  forms  like  5  and  6,  but  those  of  quite  distant 
ancestors,  as  6  and  30,  have  become  similarly  modified,  until 
they  may  resemble  one  another  so  closely  as  to  deceive  the 
casual  observer.  Forms  20  and  33,  representing  totally  dis- 
tinct stocks,  may  thus  bear  so  close  a  superficial  resemblance 
as  to  be  popularly  classed  together  under  the  same  general 
term.* 


*  Thus  whales  and  porpoises  are  vulgarly  supposed  to  be  fishes ;  shrew- 
moles,  mice;  and  bats,  birds.  Salamanders  are  usually  confused  with 
lizards;  and  certain  blind  and  limbless  lizards  (Rhineura)  which  occur  in 
Florida  and  burrow  in  the  earth,  so  closely  resemble  earth-worms  as  to 
deceive  at  first  glance  a  professional  naturalist. 


THE    CONTINUITY   OF   LIFE  21 

In  the  above  exposition  of  phylogenesis  there  can  be  seen 
at  once  both  its  advantages  and  its  disadvantages  as  an  his- 
torical record. 

In  cases  in  which  a  line  of  descent  is  well  represented  by  a 
series  of  adult  animals,  the  advantage  of  being  able  to  study 
large  forms  with  functional  parts  is  obvious;  but  where  the 
extinction  of  intermediate  forms  has  obliterated  the  record 
at  some  important  point,  the  phylogenetic  data  fail  completely 
and  must  be  supplied  by  the  parallel  history  found  in  the  indi- 
vidual development  of  the  nearest  allied  forms.  The  great- 
est assistance  has  often  been  furnished  by  palaeontology,  but 
as  the  hard  parts  alone  leave  their  imprint  in  the  rocks,  they 
are  of  little  or  no  assistance  in  the  history  of  many  of  the  sys- 
tems. Again,  through  the  metamorphosis  of  the  earlier  geo- 
logical formations  and  the  consequent  obliteration  of  all 
organic  remains  occurring  in  them,  the  palseontological  record 
has  lost  beyond  hope  of  recall  all  of  its  early  stages,  and  at  the 
period  of  the  first  fossiliferous  strata,  the  main  classes  of 
animals  as  we  have  them  at  present,  had  already  become 
established. 

It  is  here  that  the  study  of  comparative  embryology  lends 
its  assistance,  since  in  the  embryological  record  the  earliest 
stages  are  preserved,  although  often  overlaid  with  secondary 
modifications.  By  its  aid  may  be  traced,  not  only  the  lines 
connecting  any  two  forms  (Rule  II.  above),  but  it  furnishes 
faint  though  definite  clews  to  the  early  history  of  animal  de- 
velopment previous  to  the  beginning  of  the  palaeontological 
record.  Its  defects,  though  many,  are  not  the  same  as  those 
of  the  phylogenetic  record,  and  the  two  thus  reinforce  one 
another  to  a  remarkable  degree,  each  completing  the  gaps  left 
in  the  other,  and  corresponding  closely  in  those  places  in 
which  both  records  are  preserved. 

The  exposition  of  developmental  history,  or  ontogenesis, 
may  be  given  in  the  form  of  laws  as  in  the  former  case. 

I.  The  developmental  history  of  an  animal  includes  all 
stages  from  that  of  the  fertilized  egg  (ovum)  to  that  of  the 
sexually  mature  adult,  and  is  not  in  any  way  interrupted  by 


22  HISTORY    OF    THE    HUMAN    BODY 

the  act  of  birth  or  hatching.  These  latter  are  purely  external 
phenomena  and  mark  no  important  stage  in  the  development 
of  the  animal  save  in  the  line  of  certain  necessary  adaptations. 
The  birth  period  often  varies  considerably  in  allied  forms. 

These  external  phenomena  are  wholly  adaptive  and  are 
regulated  by  the  conditions  imposed  by  the  struggle  for  exist- 
ence. Thus  aquatic  salamanders  lay  eggs  which  pass  through 
all  the  stages  from  the  beginning  outside  of  the  body  of  the 
parent,  but  in  the  more  terrestrial  species,  although  closely 
allied  to  the  foregoing,  the  eggs  are  detained  in  the  oviducts 
of  the  mother,  where  development  continues  throughout  the 
larval  period  and  the  young  are  produced  in  a  practically  adult 
condition. 

It  is  advantageous  to  some  species  to  produce  a  large  num- 
ber of  immature  offspring,  relying  upon  chance  for  the  sur- 
vival of  a  few  of  them ;  under  other  circumstances  it  has  been 
proven  the  better  course  to  produce  a  small  number  of  well- 
developed  young,  furnished  with  a  better  equipment  for  fight- 
ing the  battle  of  life. 

II.  In  developmental  history  a  given  species  reproduces  in 
miniature  its  own  ancestral  history,  and  thus  passes  through 
those  stages  only  through  which  its  actual  ancestors  have  also 
passed. 

Thus,  in  the  diagram,  form  iSb  has  passed  through  the 
stages  1 8a,  18,  17,  6,  5,  4,  3,  2,  and  I  as  well  as  the  innumer- 
able stages  between  these  points  as  represented  by  the  lines 
connecting  them,  but  would  not  reproduce  any  stage  in  the 
history  of  some  allied  form  through  which  the  latter  has 
passed  since  the  divergence,  such  as  7  or  19. 

The  only  stages  common  to  any  two  recent  forms,  allied  or 
not,  are  those  below  the  point  represented  by  their  latest  com- 
mon ancestor.  This  may  be  formulated  as  follows : 

III.  In   any   two   given  forms   only   those   developmental 
stages  which  represent  common  ancestors  are  the  same  in 
both.     From  the  point  at  which  their  ancestors  diverged  their 
developmental  histories  are  distinct  and  different.     It  follows 
from  this  that  the  more  closely  allied  the  two  forms,  the  more 


THE    CONTINUITY   OF   LIFE  23 

completely  will  their  embryonic  development  coincide,  and 
conversely,  in  forms  widely  apart  the  divergence  begins  very 
early  and  only  the  first  of  the  two  developmental  histories  will 
be  coincident. 

To  illustrate  those  points:  if  the  development  of  16  and  34 
be  compared,  only  the  early  stages  i,  2  and  3  will  be  seen  to 
coincide;  if,  however,  the  developmental  histories  of  21  and 
1 6  be  taken,  they  will  be  found  coincident  as  far  as  their  last 
common  ancestor,  5.  In  closely  allied  forms,  such  as  37^  and 
37g,  almost  the  entire  embryological  history  in  the  two  animals 
will  closely  correspond,  differences  being  noted  only  at  the  last. 

IV.  The  more  highly  specialised   the  animal,    the  more 
changes  its  ancestors  have  passed  through;  and  therefore  so 
much  the  more  is  to  be  recapitulated  onto  genetically.    This  is 
effected  in  part  by  lengthening  the  embryonic  period  and  in 
part  by  sliding  over  or  dropping  out  some  of  the  stages. 

In  the  fish,  for  example,  after  the  development  of  a  simple 
circulation  designed  for  a  water-breathing  vertebrate,  there  is 
nothing  farther  to  do  than  to  perfect  and  to  mature  it  as  it  is ; 
in  the  mammal,  however,  the  circulatory  system,  which  is  at 
first  like  that  of  the  embryonic  fish,  must  become  successively 
modified  as  amphibian,  reptilian,  and  finally  mammalian;  a 
much  longer  history,  which  involves  numerous  changes  and 
adaptations. 

V.  The  different  historic  stages  are  not  given  the  same  time 
value,  but  the  earlier  the  stage,  the  more  it  is  accelerated. 
The  earlier  stages  also  lose  in  distinctness  and  detail  and  are 
more  often  lost  than  the  later  ones.     It  follows  from  this  that 
the  early  part  of  the  history  is  best  learned  from  the  lower 
forms,  in  which  the  stages  sought  are  not  very  remote  from 
the  adult  condition. 

The  approximate  time  values  of  the  developmental  stages 
are  seen  in  the  development  of  the  hen's  egg ;  the  segmentation 
stages,  and  the  formation  of  blastula  and  gastrula,  which  rep- 
resent all  the  earlier  invertebrate  portion  of  the  history,  are 
passed  through  in  a  few  hours ;  the  establishment  of  the  meso- 
dermic  somites  (myomeres),  which  makes  it  a  vertebrate,  is 


24  HISTORY    OF   THE    HUMAN    BODY 

well  marked  by  the  end  of  the  second  day ;  at  the  age  of  four 
days  the  embryo  is  sauropsidan,  at  five  or  six  definitely  avian, 
and  the  remaining  fifteen  days  are  spent  in  perfecting  the 
details  first  of  a  gallinaceous  bird,  and  lastly  of  the  particular 
species  to  which  it  belongs.  Furthermore,  the  remainder  of 
the  history,  until  the  adult  stage  is  reached,  that  is,  the  latest 
historical  period,  requires  many  months.  The  value  of  the 
study  of  the  more  primitive  forms  is  well  seen  by  the  forma- 
tion of  the  mesoderm,  and  especially  that  part  of  it  which 
give  rise  to  the  myomeres  or  primitive  muscle  segments.  In 
Amphioxus,  a  form  considerably  below  the  fishes,  the  mesoderm 
arises  from  the  primordial  intestine  in  the  form  of  paired  di- 
verticula,  from  the  dorsal  part  of  which  the  myomeres  arise; 
in  fishes  and  amphibians  these  elements  are  not  distinct 
diverticula,  but  still  possess  cavities  or  the  rudiments  of  them ; 
and  in  birds  and  mammals  the  myomeres  arise  as  solid  cubes 
cut  from  an  indifferent  cell  mass,  and  give  absolutely  no  clew 
to  their  early  history. 

VI.  In  studying  an  embryological  record  one  must  con- 
stantly distinguish  between  palin genetic  characters,  or  those 
which  are  true  repetitions  of  the  past  history,  and  cccnogenetic 
characters,  or  those  which  have  been  more  recently  acquired 
as  the  result  of  some  special  adaptation.  One  of  the  most 
universal  among  these  latter  is  the  presence  of  yolk,  a  food 
supply  for  the  embryo,  which  lies  between  or  within  the  cells 
and,  when  excessive,  causes  misleading  distortions  in  the  pro- 
portion of  parts  and  effects  the  obliteration  of  many  important 
features. 

In  general  the  actual  size  of  an  egg  is  due  to  the  amount 
of  yolk  it  contains,  and  thus  the  historic  records  are  reproduced 
with  greater  faithfulness  in  very  small  ones.  This  is  well 
shown  by  the  comparison  of  the  almost  yolkless  egg  of  Am- 
phioxus with  that  of  the  bird,  which  represents  the  other 
extreme.  In  the  one  the  cylindrical  form  of  the  primitive 
vertebrate  is  well  preserved  and  appears  almost  at  the  begin- 
ning; in  the  other  the  dorsal  portion  of  the  future  body  lies 
for  a  time  almost  flat  on  the  surface  of  an  enormous  sphere 
of  yolk,  and  is  enabled  later  to  assume  the  cylindrical  form 


THE    CONTINUITY    OF   LIFE  25 

only  through  a  secondary  adaptation  by  which  the  embryonic 
and  vitelline  (yolk)  portions  of  the  egg  become  nearly  sep- 
arated from  one  another,  the  connection  being  retained  through 
a  narrow  stalk. 

It  will  be  seen  by  the  above  exposition  of  the  two  historical 
records,  phylogenetic  and  ontogenetic,  that  they  are  by  no 
means  complete  and  that  the  fragments  that  exist  are  often 
difficult  to  interpret.  This  has  necessarily  occasioned  a  large 
amount  of  controversy  among  morphologists,  not  alone  in 
the  interpretation  of  the  facts,  but  even  in  some  cases  in  the 
recognition  of  the  facts  themselves,  owing  to  the  great  me- 
chanical difficulties  in  the  way  of  their  examination.  As  in 
all  earnest  investigation,  however,  the  differences  grow  less 
as  the  work  progresses,  and  at  the  present  time  there  is  a  prac- 
tical agreement  upon  the  main  features  of  vertebrate  history, 
the  differences  being  confined  mainly  to  details.  In  some 
cases  in  the  following  chapters  attempts  have  been  made  to 
set  forth  divergent  views,  but,  for  the  most  part,  both  for  the 
sake  of  clearness  and  in  order  to  present  the  matter  within 
suitable  limits,  the  selection  has  been  made  of  that  theory 
which,  in  the  judgment  of  the  writer,  possesses  the  greatest 
probability. 

The  significance  of  an  anatomical  fact  depends  upon  the 
phylogenetic  position  of  the  animal  studied,  yet  at  the  same 
time  it  must  be  remembered  that  the  only  criterion  we  possess 
for  making  the  phylogenetic  arrangement  is  that  of  the  an- 
atomical structure,  so  that  the  two  lines  of  investigation  are 
mutually  dependent  and  are  likely  to  become  equally  modified 
by  the  presentation  of  each  new  fact.  As  a  basis  for  this 
history  of  the  human  body,  which  is  at  the  same  time  a  history 
of  vertebrates,  especially  of  those  that  lie  in  the  direct  line  of 
human  ancestry,  it  is  thus  necessary  to  consider  the  various 
vertebrate  groups,  both  living  and  extinct,  so  far  as  we  know 
them,  and  study  their  mutual  relationships  as  deduced  from 
their  structure  and  development.  This  is,  in  fact,  a  brief  study 
of  vertebrate  phylogenesis,  and  will  be  considered  in  the  next 
chapter. 


CHAPTER  II 
THE  PHYLOGENESIS  OF  VERTEBRATES* 

"  The  Epicureans,  according  to  whom  animals  had  no 
creation,  doe  suppose  that  by  mutation  of  one  into 
another,  they  were  first  made;  for  they  are  the  sub- 
stantial part  of  the  world;  like  as  Anaxagoras  and 
Euripides  affirme  in  these  tearmes:  nothing  dieth,  but 
in  changing  as  they  doe  one  for  another  they  show 
sundry  formes." 

PLUTARCH'S  Morals;  transl.  by  Philemon  Holland, 

1603,  p.  846. 

ALTHOUGH  no  great  subdivision  of  animals,  with  the  pos- 
sible exception  of  the  echinoderms  (star-fish,  sea-urchins, 
etc.),  possesses  a  more  isolated  position  than  do  the  verte- 
brates, this  latter  group  is  connected  in  an  obscure  way  with 
the  invertebrate  world  through  a  series  of  animal  forms  of 
uncertain  position  themselves  and  usually  grouped  together 
under  the  name  of  Prevertebrata  or  Protochordata.  These 
comprise  a  worm-like  form,  Balanoglossus,  that  burrows  in 
the  mud  along  the  sea-coasts,  the  sac-like  tunlcates,  and  the 
small  and  slender  Amphioxus.  Formerly  classed  at  great 
distances  from  one  another  among  molluscs,  worms  and  even 
plants  (e.  g.,  sessile  tunicates),  they  are  now  united,  owing  to 
the  common  possession  of  pharyngeal  gill-slits,  a  dorsal 
nervous  system,  and  an  internal  skeletal  rod,  the  notochord, 
although  in  some  cases  these  two  latter  characteristics  are 
transitory  structures  that  appear  only  during  the  early  steps 
of  development. 

The  highest  of  these  animals,  and  consequently  the  one 
nearest  the  true  vertebrates,  is  Amphioxus,  a  small  marine 
creature  something  like  a  headless  fish,  which  is  found  in  the 

*  For  a  detailed  classification  of  vertebrates,  to  accompany  this  chap- 
ter, the  reader  is  referred  to  the  Appendix 


THE    PHYLOGENESIS    OF    VERTEBRATES        27 

shore  water  of  the  warmer  seas,  usually  buried  in  the  sand  in 
a  perpendicular  position,  with  the  anterior  end  projecting  into 
the  water,  expanded  into  a  sort  of  hood  for  the  collection  of 
its  food.  When  fully  grown  it  is  about  two  inches  in  length 
and  is  in  the  form  of  a  cylinder,  flattened  laterally,  and 
pointed  at  either  end.  It  is  divided  into  a  succession  of  body 
segments,  somites,  by  V-shaped  lines,  which  represent  the 
edges  of  the  partitions  of  connective  tissue,  the  myocommata. 
These  run  through  the  masses  of  body  muscles,  and  divide 
them  into  segmental  portions,  the  myomeres.  The  internal 
skeletal  axis,  which  forms  one  of  the  chief  characteristics  of 
the  group  of  vertebrates,  is  here  represented  by  a  flexible 
cylindrical  rod  of  a  substance  resembling  cartilage,  running 
through  the  body  from  tip  to  tip.  This  rod,  the  notochord, 
shows  no  trace  of  segmentation,  and  it  is  thus  seen,  as  is 
also  the  case  in  all  vertebrate  embryos,  that  the  segmentation 
so  fundamentally  characteristic  of  vertebrates,  and  so  well 
marked  in  their  internal  skeleton  (vertebrae,  ribs,  etc.),  was 
acquired  first  by  the  muscular  system,  perhaps  as  an  adapta- 
tion to  facilitate  the  flexibility  of  the  body,  and  that  it  was 
secondarily  carried  over  to  the  skeleton. 

In  arranging  a  phylogenetic  tree  of  the  vertebrates,  Amphi- 
o.nis  should  be  placed  at  the  bottom,  although,  if  absolute 
accuracy  is  demanded,  neither  Amphioxus  nor  any  modern 
animal,  with  its  later  modifications,  should  be  placed  at  any 
point  along  the  main  stems  of  the  phylogenetic  tree,  but  all 
should  be  placed  at  the  termini  of  branches ;  proximity  to  the 
ancestral  line  being  indicated  by  the  shortness  of  the  branch. 

If,  however,  later  modifications,  since  they  have  undoubt- 
edly affected  all  modern  forms  to  a  greater  or  less  extent, 
may  be  left  out  of  account,  and  if  the  successive  animal  forms 
may  be  placed  in  the  positions  occupied  by  their  direct  ances- 
tors, we  may  thus  form  a  phylogenetic  tree  like  the  one  given 
here,  which  expresses  the  relationships  of  modern  forms  to 
one  another  in  a  simple  and  essentially  correct  manner. 

Above  Amphioxus  ensues  a  great  gap,  the  greatest  in  the 
entire  series,  bridged  over  by  no  forms,  either  living  or  fos- 


28  HISTORY   OF   THE    HUMAN    BODY 

sil,  with  which  we  are  acquainted,  and  only  suggested  in  part  by 
the  members  of  the  next  higher  group,  the  cyclostomes.  This 
group  comprises  eel-like  forms,  to  be  carefully  distinguished, 
however,  from  true  eels  or  from  any  of  the  true  vertebrates, 


/PLACE.VTAL  MAMMALS 


XUS 


FIG.  7.  Phylogenetic  tree  of  vertebrates. 

Double  underscoring  indicates  an  extinct  group;  single  underscoring  one  that  has 
but  a  few  living  representatives.  The  boundaries  of  the  Classes  are  represented  by 
dotted  lines. 

since  they  possess  neither  jaws  nor  teeth  in  the  sense  of  those  of 
the  higher  vertebrates,  but  have  the  mouth  surrounded  by  a 
circular  lip  which  is  capable  of  being  extended  so  as  to  re- 
mind one  of  the  hood  possessed  by  Amphioxus.  Within  this 
mouth  there  are  variously  shaped  spines  or  plates  which  serve 


THE    PHYLOGENESIS    OF   VERTEBRATES        29 

as  teeth.  A  most  important  distinction  between  Amphio.rus 
and  the  cyclostomes,  however,  lies  in  the  fact  that  the  latter 
possess  a  definite  head,  with  brain  and  sense  organs,  parts 
which  exist  only  in  a  rudimentary  or  potential  sense  in  Am- 

phlOJCUS. 

In  distinction  from  the  cyclostomes,  or  "  round-mouths," 
are  the  true  vertebrates,  which  are  termed  gnathostomes,  or 
"  jaw-mouths,"  the  possession  of  jaws  being  a  constant  char- 
acteristic of  the  entire  group.  The  lowest  class  of  gnathos- 
tomes is  that  of  the  fishes  (Pisces),  but  these  are  in  turn 
subdivided  into  several  groups,  some  of  which  represent 
lateral  branches,  that  is,  specializations  along  definite  direc- 
tions, and  thus  not  in  the  direct  line  of  human  history.  The 
most  primitive  group  is  that  of  selachians,  which  comprises 
the  sharks  and  dog-fish,  and  the  skates  or  rays.  This  group 
of  animals  is  absolutely  fundamental  for  the  morphologist 
and  represents  the  first  great  stage  in  the  main  line  of  verte- 
brate history.  Selachians  have  a  wholly  cartilaginous  skele- 
ton, the  mouth  upon  the  lower  side  of  the  head  and  not  at  the 
anterior  end,  as  in  other  fish,  and  five  gill-slits  which  open 
separately  and  free,  not  covered  by  an  operculum  (gill-flap). 
Their  position  in  the  tree  is  clearly  in  the  main  line  above 
the  cyclostomes. 

The  ganoid  fishes  are  also  of  great  importance  to  us. 
They  represent  a  T:ew  remnants  of  what  was  the  dominant 
group  during  the  Devonian  epoch  and  are  the  direct  de- 
scendants of  the  selachians.  As  in  the  case  of  all  such  rem- 
nants, they  are  extremely  diverse  in  structure  among  them- 
selves and  are  placed  in  a  single  group  rather  more  for  con- 
venience than  because  of  a  very  close  relationship  to  one 
another.  They  are  characterized  by  the  tendency  of  the 
scales  to  fuse  into  bony  plates,  a  tendency  which  in  the  past 
resulted  in  the  development  of  a  special  group,  -the  placo- 
dcrms,  which  were  entirely  covered  by  a  suit  of  mail  formed 
in  this  way.  Similar  plates  cover  the  head  in  all  modern 
ganoids  and  they  occur  in  rows  along  the  body  in  a  few 
forms  (sturgeons).  The  skeleton  is  mainly  cartilaginous  in 


30  HISTORY    OF    THE    HUMAN    BODY 

the  lower  representatives  of  this  group,  but  becomes  more  or 
less  bony  in  the  higher.  The  gill-slits  no  longer  open  directly 
and  separately  to  the  outside,  as  in  their  selachian  ancestors, 
but  are  grouped  together  and  covered  by  a  gill-flap  or  oper- 
culum. 

The  two  remaining  groups  of  fishes,  teleosts  and  dipnoans, 
represent  independent  lateral  branches  that  have  specialized 
in  accordance  with  certain  definite  lines  and  are  consequently 
not  in  the  direct  line  of  man's  ancestry.  Such  groups  often 
form  collateral  testimony  of  considerable  morphological  value 
and  are  thus  not  without  importance  even  in  the  present  line 
of  speculation.  The  teleosts  have  an  almost  completely  ossi- 
fied skeleton  and  are  the  descendants  of  the  bony  ganoids,  with 
which  they  are  so  closely  connected  through  intermediate 
forms  that  the  separation  between  them  is  mainly  an  artificial 
one.*  They  are  essentially  a  modern  group  and  constitute 
the  great  majority  of  the  fishes  in  the  world  to-day,  thus 
taking  the  place  of  the  ganoids  of  earlier  times.  The  dipnoi 
are  represented  by  but  three  forms,  one  found  in  Africa,  one 
in  Australia  and  one  in  South  America.  They  are  fresh-water 
fishes  and  are  remarkable  for  their  power  of  sustaining  long 
periods  of  drought  by  digging  into  the  mud,  and  breathing 
air  through  a  modified  air-bladder.  They  were  thus  for- 
merly considered  the  link  between  fishes  and  amphibians,  but 
later  researches  into  their  structure  do  not  confirm  this  view. 

As  a  matter  of  fact  the  amphibians  seem  to  have  come  from 
the  ganoids,  although  by  means  of  forms  now  lost,  and  to  have 
developed  first  into  the  Stegocephali,  a  group  wholly  extinct 
but  well  represented  by  fossil  remains  occurring  in  and  about 
the  coal  deposits.  These  had  many  of  the  characteristics  of 
our  modern  amphibians,  but  possessed  scales  arranged 
in  definite  rows,  organs  which  are  entirely  lacking  in  all 
living  representatives  of  this  Class,  with  the  exception  of  the 

*  Although  the  employment  of  the  two  terms  "  ganoid  "  and  "  teleost " 
is  a  convenient  one  in  comparative  anatomy,  modern  ichthyologists  tend 
strongly  to  the  rejection  of  both  terms  and  the  fusion  of  the  two  groups 
into  a  single  one,  the  Teleostomi.  Cf.  Appendix. 


THE    PHYLOGENESIS    OF   VERTEBRATES        31 

Gymnophiona,  which  still  possess  scale  rudiments,  not  visible 
externally.  The  Stegocephali  are  of  extreme  importance,  since 
they  were  the  ancestors  both  of  the  present-day  amphibians 
and  of  the  two  main  reptilian  lines,  and  the  survival  of  a  single 
representative  would  have  been  of  priceless  value  to  mor- 
phologists.  As  it  is,  however,  we  are  in  possession  of  a  large 
number  of  fossil  remains,  many  of  them  extremely  well  pre- 
served, and  representing  four  distinct  orders ;  and  further  dis- 
covery along  this  line  may  well  be  expected  at  any  time.  Of 
the  soft  parts  the  fossil  imprints  furnish  but  little  evidence, 
a  lack  which  must  be  supplied  by  the  study  of  the  urodeles, 
undoubtedly  their  nearest  living  allies  and  presumably  not  very 
different  in  the  essential  internal  features. 

These  latter  animals,  though  not  quite  in  the  direct  line 
of  human  ancestry,  are  thus  of  the  greatest  importance  as  the 
best  representatives  of  what  may  be  called  the  amphibian 
stage.  The  urodeles  comprise  the  tailed  amphibians,  their 
most  typical  representatives  being  the  forms  known  as  sala- 
manders and  newts,  also  in  many  sections,  unfortunately, 
"  lizards,"  owing  to  their  superficial  resemblance  to  these 
latter  animals.  The  more  primitive  members  of  this  group 
are  often  large  (10-40  cm.),  and  the  giant  Cryptobranchus  of 
Japan,  the  largest  of  all  living  amphibians,  attains  the  length 
of  a  meter. 

The  Anura,  or  tailless  amphibians,  include  frogs,  toads  and 
tree-toads,  and  attain  their  tailless  condition  in  part  by  a 
retrogressive  development  of  the  caudal  region  and  in  part 
through  the  excessive  development  of  the  ilia  and  the  thigh 
muscles,  a  feature  connected  with  their  jumping  habits.  The 
Gymnophiona  are  blind  subterranean  forms,  burrowing  in  the 
earth  like  earth-worms,  to  which  they  bear  considerable  re- 
semblance. They  are  much  attenuated,  are  without  external 
limbs,  and  have  their  bodies  clearly  marked  off  into  annular 
segments.  They  occur  only  in  the  warmer  parts  of  the  world 
and  consist  of  but  few  forms. 

Arising  also  from  the  Stegocephali  come  the  reptiles,  which 
have  apparently  developed  along  two  lines,  the  one  leading  to 


32  HISTORY   OF   THE    HUMAN    BODY 

the  birds,  the  other  to  the  mammals.  Of  the  first  of  these,  the 
oldest  group  is  that  of  the  Rhyjiekocephalia,  mainly  fossils, 
but  with  a  single  living  species,  which  fate  has  preserved  in 
New  Zealand,  the  Sphenodon  (Hatteria).  This  represents 
the  ancestor  of  lizards  and  snakes,  Lacertilia  and  Ophidia  re- 
spectively, and  also  a  group  of  extinct  reptilian  giants,  the 
dinosaurs,  whose  nearest  living  allies  are  the  crocodiles.  Here 
this  line  would  have  ended,  so  far  as  human  knowledge  is 
concerned,  had  it  not  been  for  the  chance  discovery,  about 
the  middle  of  the  nineteenth  century,  of  two  specimens  of  one 
of  the  most  remarkable  "  missing  links "  ever  found,  the 
Archceopteryx,  a  form  midway  between  reptiles  and  birds,  and 
of  undoubted  affinity  to  the  stem  of  the  dinosaurs.  This 
creature  was  bird-like,  possessed  wings  and  a  certain  number 
of  contour  feathers,  but  had  a  long  vertebrated  tail,  several 
free  digits  in  the  hand,  furnished  with  curving  claws,  and  a 
heavy  jaw  containing  conical  teeth,  reptilian  in  character.  This 
discovery,  followed  by  that  of  the  toothed  birds,  completed 
the  chain  of  evidence,  and  supplied  one  of  the  most  isolated 
groups  of  vertebrates  with  a  definite  line  of  ancestry. 

The  other  line  of  reptiles,  which  may  have  arisen  from  the 
Stegocephali  more  or  less  independently  of  the  first,  was  that 
beginning  with  the  theromorphs,  an  extinct  group,  many  of 
which  attained  a  gigantic  size.  Some  members  of  this  group 
are  so  near  the  mammals  in  many  particulars  that  it  has  been 
only  with  the  greatest  care,  and  through  the  consideration  of 
all  the  available  parts,  that  their  reptilian  nature  has  been  de- 
termined. In  studying  the  remains  of  these  forms,  especially 
those  of  the  sub-group  of  theriodonts,  the  most  of  which  were 
small  animals,  like  the  earliest  mammals,  it  seems  impossible 
not  to  assign  them  a  close  relationship  to  the  latter,  probably 
that  of  actual  ancestry.  Indeed,  there  is  at  present  but  one 
other  claimant  for  that  position,  and  that  is  the  group  of 
Stegocephali,  and  as  these  were  contemporary  with  the 
theromorphs,.  and  at  one  time  probably  graded  into  them 
by  imperceptible  transitions,  the  two  views  are  not  very  wide 
apart.  All  things  considered,  it  seems  that  the  gap  between 


THE    PHYLOGENESIS    OF   VERTEBRATES        33 

Stegocephali  and  the  mammals  requires  some  intermediate 
link,  and  thus  the  addition  of  the  theromorphs  in  this  place 
seems  rather  a  completion  than  an  opposition  to  the  theory 
of  Stegocephalan  ancestry. 

The  only  living  reptiles  associated  with  the  same  branch  as 
the  theromorphs  are  the  turtles  (Chelonia),  which,  although 
highly  specialized  in  the  matter  of  trunk  skeleton,  are  of  the 
greatest  value  in  regard  to  their  soft  parts,  which  are  un- 
doubtedly similar  to  those  of  the  extinct  members  of  the 
branch,  and  are  thus  the  best  living  representatives  of 
the  important  stage  between  amphibians  and  the  early 
mammals. 

The  earliest  mammalian  remains  are  contemporary  with 
those  of  the  theromorphs,  and  are  those  of  small  forms,  like  the 
most  mammalian  of  the  reptilian  remains.  These  are  ap- 
parently nearly  related  to  the  monotremes,  the  lowest  living 
mammals,  which  are  represented  by  two  forms  occurring  in 
Australia  and  New  Zealand,  the  Duck-bill  Platypus  (Ornith- 
orhynchus)  and  the  spiny  ant-eater  (Echidna).  The  latter 
has  no  connection  with  the  true  ant-eaters  (Myrmecophagida) 
of  South  America,  which  are  placental  mammals.  The  mono- 
tremes  are  strongly  reptilian  in  certain  skeletal  features;  like 
true  reptiles  and  unlike  all  other  mammals,  they  possess  a 
single  terminal  orifice,  that  of  a  common  cloaca,  into  which 
open  the  alimentary  canal,  the  ureters  and  the  genital 
ducts;  and  they  actually  lay  eggs,  that  is,  very  immature  em- 
bryos, surrounded  by\  a  thin,  cornified  shell.  The  mammary 
glands,  one  of  the  essential  characteristics  of  the  class  of  mam- 
mals, are  seen  here  in  a  very  simple  condition.  They  consist  of 
two  lateral  groups  of  integumental  glands,  apparently  of  the 
tubular  type,  which  open  separately  in  the  bottom  of  an  oval 
depression,  the  mammary  pocket.  There  are  no  teats,  and 
the  young  obtain  the  secretion  either  directly  from  the  de- 
pressions or  by  sucking  at  the  hair  in  this  region. 

The  next  group  above  the  monotremes  are  the  marsupials, 
with  the  exception  of  the  opossum  also  confined  to  the  Aus- 
tralian region.  As  in  the  previous  group,  the  young  are  born 


34  HISTORY    OF   THE    HUMAN    BODY 

in  an  immature  state,  but  are  unprotected  by  an  egg-shell,  and 
are  matured  in  an  external  abdominal  pouch (marsupium) until 
able  to  care  for  themselves.  The  relation  between  the  mono- 
tremes  and  the  modern  marsupials  is  hardly  close  enough  to 
justify  an  immediate  succession,  but  suggests  that  each  group, 
as  we  now  know  it,  has  descended  from  more  primitive  an- 
cestors that  were  thus  related;  that  is,  that  the  ancestor  of 
modern  marsupials  was  a  direct  descendant  of  the  ancestor  of 
the  monotremes. 

Beyond  the  marsupials  all  the  mammals  are  placenta!,  that 
is,  the  embryos  are  retained  for  a  longer  time  within  the 
uterus  of  the  parent  and  are  nourished  by  means  of  an  organ 
formed  in  part  from  the  mucous  membrane  of  the  uterus  and 
in  part  from  tissue  furnished  by  the  embryo  but  not  included 
within  its  body.  This  organ  is  termed  the  placenta  and  is 
connected  with  the  body  of  the  embryo  through  an  umbilical 
cord.  This  cord  contains  fetal  blood  vessels  which  connect 
proximally  with  the  main  circulatory  system  of  the  embryo 
and  develop  distally  into  a  system  of  capillaries  that  lie  in  villi 
in  the  embryonal  portion  of  the  placenta,  obtaining  their 
nourishment  and  effecting  the  interchange  of  respiratory  gases 
through  osmotic  transmission.  There  is  thus  no  direct  organic 
continuity  between  mother  and  offspring,  and  neither  nerves 
nor  blood  vessels  are  continuous  from  one  to  the  other.  In- 
deed, in  the  lower  placental  mammals  the  connection  between 
the  maternal  and  embryonal  portions  of  the  placenta  is  very 
loose  and  the  two  easily  separate  at  birth,  although  in  the 
higher  forms  the  connection  becomes  more  intimate  and  the 
separation  takes  place  between  the  muscular  and  mucous  coat 
of  the  uterus,  thus  involving  an  actual  loss  of  maternal 
tissue. 

The  placental  mammals,  although  their  appearance  was  com- 
paratively recent,  geologically  speaking,  have  specialized  in  all 
directions,  and  now  occupy  almost  every  available  environment, 
not  only  of  the  land,  but  of  the  water.  Some  are  fitted  to 
pursue  and  drag  down  large  herbivorous  animals,  while  others 
feast  upon  dead  bodies  or  suck  the  blood  of  the  living  after 


THE    PHYLOGENESIS    OF   VERTEBRATES        35 

the  manner  of  parasites.  Many  are  specially  adapted  to  the 
capture  of  insects,  either  on  or  beneath  the  surface  of  the 
ground,  or  on  trees,  and  some  have  even  developed  the  power 
of  flight  by  which  they  may  follow  their  prey  through  the 
air.  The  hosts  of  the  vegetable  feeders  are  as  highly  dif- 
ferentiated and  become  specially  adapted  to  feed  either  upon 
low  herbage  or  the  leaves  of  trees,  roots,  bark  or  fruits,  and 
have  even  developed  one  group  of  oceanic  forms,  fitted  to 
browse  upon  the  sea- weeds  and  other  submerged  vegetation. 

These  various  lines  of  specialization,  together  with  the  usual 
extinction  of  intermediate  forms,  have  produced  a  series  of 
more  or  less  isolated  groups,  or  Orders,  the  interrelationships 
of  which  have  been  deciphered  in  part  by  the  labors  of  anat- 
omists, in  part  by  those  of  palaeontologists,  but  are  still  more 
or  less  uncertain.  A  suggestion  of  this  is  shown  in  the  ac- 
companying phylogenetic  tree  of  mammals  (Fig.  8.),  which 
takes  into  consideration  both  living  and  extinct  groups,  so 
far  as  known. 

The  earliest  mammalian  forms,  of  which  we  possess  only 
fragmentary  remains,  were  more  like  the  reptiles,  and  espe- 
cially the  theromorphs,  than  any  now  extant,  but  possessed 
many  of  the  characters  of  the  monotremes,  which  may  be  con- 
sidered their  somewhat  highly  specialized  descendants.  To 
this  group  has  been  given  the  name  Pantotheria,  and  as  the 
ancestors  of  all  the  rest  they  may  form  the  main  trunk  of 
the  phylogenetic  tree.  The  monotremes  are  the  nearest  living 
descendants,  and  they  have  been  derived  from  them  through 
an  ancient  and  closely  related  group,  the  Multituberculata. 
All  three  of  these  groups  were  reptilian  in  structure,  and  may 
be  classed  together  and  in  contrast  to  all  the  other  mammals, 
as  the  Sub-class  Prototheria. 

\Yhile  still  primitive,  however,  the  Pantotheria  began  to 
differentiate  along  two  lines,  the  one  somewhat  resembling  the 
marsupials,  the  other  the  insectivores,  and  thus  early  these 
two  lines  of  development  became  inaugurated.  Eventually  the 
reptilian  characters  were  dropped,  and  the  animals,  passing 
over  into  the  Sub-class  Eutheria,  or  typical  mammals,  be- 


HISTORY   OF   THE    HUMAN    BODY 


came  respectively  the  Didelphia,  or  marsupials,  and  the  Mono- 
delphia,  or  placentals.     The  first  of  these  lines  then  differ- 


PROTOTHERIA 


EUTHERIA 


FIG.  8.  Phylogenetic  tree  of  mammals. 

A  branch  that   terminates   in  an   arrow   point  still   possesses  living   representativesr, 
one   that    ends   in  a   short  cross   bar   is   extinct. 

entiated  into  the  marsupialian  Orders  of  the  present  time,  dis- 
tinguished mainly  by  variation  in  the  dentition,  and  the  second, 


THE    PHYLOGENESIS    OF   VERTEBRATES        37 

which  resembled  the  present-day  Insectivora,  passed  over  into 
that  Order. 

This  insectivorous  stem,  in  addition  to  perfecting  its  own 
type  along  the  narrow  lines  first  laid  down,  developed  several 
lines  of  differentiation,  and  it  was  from  these  that  all  the 
higher  placental  mammals  have  arisen.  A  very '  primitive 
stem  is  that  of  the  Rodentia,  of  which  the  extinct  group  of 
Tillodontia  may  have  been  the  first;  succeeded  by  the  Du- 
plicidentata  or  gnawing  animals,  like  the  rabbits,  in  which, 
back  of  the  two  sharp  upper  incisors,  there  is  a  second  re- 
duced pair,  and  later  by  the  Simplicidentata,  like  squirrels, 
rats,  mice,  and  beavers,  in  which  the  upper  incisors  consist 
of  a  single  pair. 

The  branch  represented  here  as  immediately  above  the  last, 
suggesting  a  little  less  primitive  character,  is  that  leading  to 
the  group  usually  called  the  Edentata,  and  consisting  of  the 
sloths,  armadilloes,  ant-eaters,  besides  several  extinct  forms, 
such  as  the  Megatherium,  Megalonyx,  and  Glyptodon,  the 
first  two  like  the  sloths,  the  last  like  an  armadillo.  In  the 
more  specialized  of  these  there  is  a  peculiar  joint  between 
two  of  the  vertebrae  of  the  back,  and  they  are  called  the  Xenar- 
thra  in  contrast  to  those  in  which  this  joint  is  normal,  the  No- 
marthra.  This  group  has  always  been  exclusively  American, 
the  living  forms  mainly  South  American. 

From  this  same  generalized  group,  the  Insectivora,  there 
have  developed  two  distinct  lines  of  flying  or  soaring  forms, 
the  Chiropfera  or  bats,  and  the  Galeopithecus,  a  single  species 
found  in  Madagascar,  but  not  nearly  related  to  any  of  the 
other  stems. 

By  far  the  most  prolific  of  the  stems  proceeding  from  the 
Insectivora  is  that  which  started  with  the  extinct  group  of 
Creodonta.  These  animals  were  at  first  small,  generalized 
mammais,  scarcely  distinguishable  from  the  parent  insecti- 
vores,  but  they  gradually  took  on  special  characters  which 
suggest  the  modern  Carnivora,  which  are  considered  their 
direct  descendants.  Before  specializing  along  this  line,  how- 
ever, some  of  them  began  to  differentiate  in  several  other  di- 


38  HISTORY   OF   THE    HUMAN    BODY 

rections  and  thus  gave  origin  to  the  Primates,  the  Condy- 
larthra,  a  generalized  form  of  ungulate,  and  probably  a  line 
of  aquatic  carnivorous  forms,  destined  to  become  the  most 
erratic  and  singular  of  all  mammals,  the  Cetacea,  or  whales 
and  porpoises.  These  earliest  ancestors  of  divergent  lines 
were  very  much  alike,  and  the  early  primate,  carnivorous, 
and  hoofed  forms,  were  all  very  generalized,  and  without  the 
differential  characteristics  that  their  descendants  later  de- 
veloped. The  most  primitive  of  the  Primates  were  a  group 
called  the  Mesodonta,  of  which  the  modern  lemurs  are  the 
most  direct  descendants.  Very  early,  however,  forms  like  the 
modern  monkeys,  Anthropoidea,  began  to  make  their  appear- 
ance, forms  in  which  the  orbit  was  entirely  separated  from 
the  temporal  fossa,  and  in  which  the  dentition  was  the  same 
as  in  the  monkeys  of  the  Old  World  and  in  Man ;  and  in  these 
we  find  the  direct  human  ancestors. 

The  creodont  stem  developed,  as  stated  above,  the  modern 
Carnivora,  including  the  cats,  dogs,  bears,  and  weasels,  and 
from  this,  at  an  early  date,  there  probably  arose  a  carnivorous 
line  that  adapted  itself  to  the  sea.  This  is  the  Pinnipedia,  or 
those  with  fin-like  feet,  the  seals,  the  walrus,  sea-lion,  etc. 

The  remaining  stem,  that  of  the  Condylarthra,  was  per- 
haps the  most  prolific  of  all  in  respect  to  the  amount  of  vari- 
ation, and  the  extent  of  modification,  for  it  has  produced  the 
Sirenia,  aquatic  forms,  nearly  as  highly  specialized  as  the 
whale ;  the  Proboscidia,  or  elephants,  with  an  excessive  modifi- 
cation of  the  nose;  and  an  enormous  variety 'of  animals 
with  a  reduction  of  toes,  the  series  reaching  its  absolute  limit 
in  the  horse,  which  has  lost  all  the  digits  but  one,  this  be- 
coming greatly  strengthened  to  serve  the  purpose  of  an  en- 
tire foot. 

•  The  original  Condylarthra  have  long  been  extinct,  as  well 
as  the  earlier  derivatives,  the  Amblypoda,  Ancylopoda,  Taxe- 
opoda,  and  Litopterna;  but,  fortunately,  of  all  these  there  is 
left  a  single  solitary  Genus,  Procavia  or  Hyrax,  for  which 
the  Order  Hyracoidea  has  been  made.  This  is  a  little  animal 
of  about  the  size  of  a  rabbit ;  the  one  referred  to  in  the  King 


THE    PHYLOGENESIS    OF   VERTEBRATES        39 

James  Bible  as  the  "  coney."  It  frequents  Syria  and  the  ad- 
joining countries,  and  a  related  species  is  found  in  South 
Africa ;  the  sole  survivors  of  the  early  ungulates. 

The  modern  ungulate  forms,  aside  from  Hyrax,  may  be 
represented  by  two  stems,  the  one  leading  to  the  Proboscidea 
and  Sirenia,  the  other  branching  immediately  into  the  Peris- 
sodactyla,  with  an  odd  number  of  functional  digits,  and  the 
Artiodactyla,  with  an  even  number.  The  Proboscidea  include 
the  two  species  of  living  elephants,  besides  several  extinct 
ones,  like  the  mammuth,  the  mastodon,  and  the  dinotherium, 
and  the  Sirenia  consist  of  two  living  genera  of  unwieldly 
aquatic  herbivores,  the  manatee  or  sea-cow,  and  the  dugong, 
which  subsist  on  sea-weeds  and  consequently  do  not  wander 
far  from  the  coasts.  The  Perissodactyla  include  the  three 
lines  represented  by  the  tapir,  the  rhinoceros,  and  the  horse; 
and  the  Artiodactyla  embrace  the  non-ruminant  pigs  and 
hippopotami,  and  the  almost  numberless  species  of  ruminants, 
such  as  cattle,  sheep,  antelopes,  and  deer.  Of  these  perhaps  the 
most  distinct  are  the  giraffes,  and  the  Tylopoda,  or  camels. 

In  reviewing  the  two  phylogenetic  trees  as  given  in  Figs. 
7  and  8,  it  will  be  seen  that  it  is  precisely  those  forms  that 
are  the  most  needed  to  show  the  interrelationships  of  groups 
that  have  suffered  the  most  from  the  extinction  of  their  species, 
which  is  but  another  way  of  expressing  the  fact  that  general- 
ized and  transition  forms  are  not  as  well  fitted  for  the  struggle 
for  existence  as  are  their  more  specialized  and  better  adapted 
descendants,  and  are  hence  often  exterminated  by  the  very 
races  which  have  developed  from  them.  This  extermination 
tends  to  isolate  the  terminal  groups  and  thus  to  disguise  the 
plan  of  development,  as  may  be  seen  by  reference  to  Fig.  7, 
in  which  the  distinction  is  shown  between  living  and  extinct 
groups.  The  effect  of  extinction  will  here  be  shown  if  the 
reader  imagines  the  extinct  groups  completely  blotted  out, 
which  will  leave  the  modern  orders  entirely  cut  off  from  one 
another. 

The  same  principle  may  be  seen  also  in  the  second  diagram, 
the  phylogenetic  tree  of  mammals  (Fig  8).  Here  the  groups 


40  HISTORY   OF   THE    HUMAN    BODY 

of  the  greatest  importance  in  showing  relationships  are  the 
primitive  Insectivora,  the  Mesodonta,  the  Condylarthra,  and 
the  Creodonta,  and  although  the  existence  of  the  first  could 
be  surmised  from  their  modern  descendants,  the  discovery  of 
the  fossil  remains  of  the  others  were  absolutely  essential  to 
the  reconstruction  of  the  original  relations  between  the  three 
great  groups  of  primates,  carnivores,  and  ungulates.  It  is 
thus  not  surprising  that  the  various  orders  of  mammals  have, 
until  recently,  been  treated  like  isolated  groups,  and  that,  even 
yet,  any  scheme  that  may  be  offered  must  be  looked  upon  as 
provisional  and  liable  to  be  modified  by  the  bringing  to  light 
of  new  evidence,  especially  that  from  palseontological  sources. 
It  will  be  noticed  that  the  branch  leading  to  the  Primates, 
the  order  to  which  Man  belongs,  is  represented  in  the  dia- 
gram as- one  of  the  shortest  and  least  specialized,  a  presentation 
which,  although  opposed  to  the  prevailing  opinion,  is  in  strict 
accord  with  the  facts ;  since  in  anatomical  structure  these  ani- 
mals show  comparatively  little  deviation  from  the  primitive 
mammalian  type  and  do  not  exhibit  the  extreme  specialization 
displayed  by  the  groups  representing  most  of  the  other  terminal 
branches.  Such  aberrant  orders  as  those  of  the  bats,  whales, 
and  horses,  which  have  departed  farthest  from  the  original 
mammalian  environment,  show  in  consequence  the  greatest 
modifications  and  are  thus  the  most  specialized;  certain  other 
groups,  the  peculiarities  of  which  are  not  so  striking,  are  still 
greatly  modified  in  comparison  with  the  Primates.  Thus  the 
majority  of  the  ungulates  show  a  reduction  in  the  original 
number  of  digits,  the  extremes  resulting  in  either  two,  as  in 
the  camels  and  deer,  or  one,  as  in  the  horse ;  but  the  Primates, 
together  with  the  rodents  and  modern  insectivores,  preserve 
the  original  number  of  five,  inherited  directly  from  the  am- 
phibians and  reptiles.  The  teeth  of  ungulates  are  character- 
ized by  a  great  complexity  in  the  folding  of  the  enamel  layer, 
and  in  the  number  and  arrangement  of  the  cusps ;  those  of  ro- 
dents are  specialized  for  the  purpose  of  gnawing,  and  in  the 
Cetacea  they  are  either  secondarily  reduced  to  the  form  of 
simple  cusps,  all  alike,  or  are  lost  altogether;  the  Primates, 


THE    PHYLOGENESIS    OF   VERTEBRATES        41 

however,  are  very  simply  constructed  in  these  particulars 
and  remain  close  to  the  lower  type  as  shown  in  the  marsupials. 
Primates  are  also  primitive  in  their  muscular  system,  pos- 
sessing in  many  instances  a  single  undifferentiated  muscle- 
mass  where  the  members  of  other  Orders  show  a  complex 
group  of  muscular  units. 

Aside  from  the  adaptation  of  their  extremities  to  an  ar- 
boreal life,  the  one  line  of  development  by  which  the  Primates 
have  become  differentiated  is  in  that  of  their  central  nervous 
system,  and  especially  that  of  the  cerebrum,  which  has  given 
them  a  far  greater  capacity  for  recording  their  sensory  im- 
pressions, and  thus  of  profiting  by  experience,  the  basis  for  the 
development  of  reason.  It  is  chiefly  in  this  respect  that  the 
human  species  has  developed  so  far  beyond  the  condition  of 
the  other  Primates  that  the  world  has  long,  and  perhaps  'will- 
ingly, been  deceived  in  regard  to  their  true  relationship.  In 
spite  of  all  prejudice,  however,  man  is,  anatomically  speak- 
ing, a  typical  primate,  closely  related,  even  in  many  of  the 
smaller  details,  to  the  rest,  and  the  only  way  in  which  he  has 
proved  superior,  through  the  excessive  development  of  the 
cerebral  hemispheres,  is  not  a  modification  calculated  to  pro- 
duce important  correlated  changes  in  the  other  parts.  Of 
the  two  living  Sub-orders,  the  Lemur oidea  and  the  Anthro- 
poidea,  the  former  are  the  more  primitive  and  more  nearly 
represent  the  generalized  Mesodonta  from  which  the  race 
sprung.  In  the  completeness  of  the  partition  which  separates 
the  orbital  and  temporal  fossae,  Man  is  seen  to  be  an  Anthro- 
poid; and  in  important  characters,  such  as  the  reduction  of 
the  premolars  from  three  to  two,  he  agrees  with  the  Catarrhine 
division  of  this  Sub-order.  If  we  employ  the  usual  schedule 
of  values  to  be  attached  to  points  of  structural  difference,  as 
used  for  the  purpose  of  classification,  we  cannot  fairly  place 
him  in  a  Family  apart  from  the  large  tailless  apes  of  the  Old 
World,  and  aside  from  this  we  have  several  intermediate 
links,  which  the  researches  of  the  past  few  years  have  brought 
to  light,  and  which  reduce  even  the  slight  gap  formerly  con- 
sidered to  be  between  them. 


42  HISTORY    OF    THE    HUMAN    BODY 

The  date  of  Man's  appearance  on  the  earth  has  been  pushed 
back  many  thousands  of  years  beyond  what  was  formerly  be- 
lieved to  be  possible,  and  this  has  been  absolutely  proven  by 
the  most  indisputable  facts.  Crania  of  the  present  human  type 
have  been  discovered  in  Europe  in  association  with  the  re- 
mains of  such  extinct  forms  as  the  cave-bear  and  the  hairy 
mammuth,  and  numerous  carvings  and  incised  drawings  have 
been  discovered  in  which  the  latter  animal  has  been  por- 
trayed by  an  eye-witness  and  with  much  artistic  ability.  This 
brings  the  present  species,  Homo  sapiens,  with  proportions 
like  that  of  the  modern  European,  back  to  the  end  of  the  last 
glacial  epoch,  or,  as  some  think,  to  a  time  contemporary 
with  it. 

Aside  from  this,  there  have  also  been  found,  dating  from 
about  the  same  period,  remains  of  men,  or  man-like  creatures, 
of  proportions  unknown  at  the  present  time  and  constituting 
a  distinct  species,  Homo  primigenius  (H.  neanderthal  ensis). 
Such  remains  have  been  found  at  Spy  in  Belgium,  in  the 
Neanderthal  near  Diisseldorf,  at  Cannstadt  in  Prussia,  in 
the  bed  of  the  river  Liane  near  Boulogne-sur-mer,  and  in 
other  localities,  the  specimens  all  closely  corresponding  to  one 
another  and  equally  unlike  the  present  living  species.  The 
forehead  and  cranium  of  the  "  Neanderthal  man,"  as  seen 
from  these  specimens,  was  extremely  low  and  flat,  and  the 
superciliary  ridges  above  the  eyes  were  so  heavy  and  promi- 
nent that  they  formed  together  a  pair  of  projecting  arches 
hung  over  the  deep-set  eyes.  There  was  almost  no  chin.  The 
height  was  that  of  a  rather  small  man;  the  arms  were  not 
excessively  long,  but  the  thigh-bones  were  permanently  curved 
and  the  tibiae  were  short,  so  that  an  absolutely  erect  position 
was  impossible.  In  spite  of  the  general  ape-like  appearance 
and  the  low  character  of  the  cranium,  the  actual  capacity  of 
the  latter  was  about  that  of  a  modern  Australian,  and  the 
presence  of  flint  implements  in  association  with  the  remains 
show  that  this  species  could  lay  claim  to  being  termed  a  man 
although  of  a  distinct  type  from  the  one  that  has  survived. 

The   fossil   remains  of  an  animal,   in  many  respects  pre- 


THE    PHYLOGENESIS    OF    VERTEBRATES        43 

cisely  intermediate  between  Homo  primigenius  and  the  an- 
thropoid apes,  were  discovered  in  Java  in  1891  in  deposits  of 
the  late  Tertiary  period,  and  were  named  Pithecanthropus 
erectus,  the  generic  name,  "  ape-man,"  having  been  proposed 
some  years  before  for  the  then  hypothetical  transition  form,  the 
"  missing  link  "  of  popular  fancy.  These  remains  consist  of 
a  cranium,  a  femur  and  three  molar  teeth,  and  although  not 
found  in  contact  with  one  another,  their  relation  to  their 
surroundings  was  such  as  to  declare  them  the  -fragments  of  a 
single  skeleton. 

In  the  cranium  the  ape-like  characters  seen  in  Homo  primi- 
genius are  here  still  more  pronounced ;  the  cranial  vault  is  still 
lower,  the  superciliary  ridges  are  still  more  prominent,  closely 
approximating  those  of  a  chimpanzee  or  gibbon.  The  pro- 
portions of  the  teeth  suggest  a  dental  arcade  intermediate  be- 
tween the  flattened  form  seen  in  man  and  the  elongated  arch 
of  the  living  anthropoids;  the  probable  shape  of  the  tongue 
and  hard  palate,  as  deduced  from  this,  would  seem  to  have 
allowed  the  production  of  many  of  the  more  elementary  sounds 
occurring  in  human  speech.  An  independent  fact  that  corrob- 
orates this  conclusion  was  determined  later  when  the  con- 
figuration of  the  brain  surface  was  obtained  by  means  of  a 
cast  of  the  interior  of  the  cranium,  for  here  the  center  of 
articulate  speech  (the  left  lower  frontal  convolution)  was 
found  to  have  been  more  developed  than  in  the  highest  apes 
but  considerably  less  so  than  in  man. 

The  femur  does  not  exhibit  the  transitional  characters 
which  one  would  be  led  to  expect  from  the  nature  of  the 
cranium,  for  it  is  essentially  human  in  form  and  shows  a 
higher  type  than  that  of  the  European  Homo  primigenius. 
Pithecanthropus  must  thus  represent  a  parallel  or  collateral 
form  in  which  the  development  in  the  direction  of  an  erect 
position  had  reached  a  high  plane  while  the  cranium  and 
brain  remained  at  a  stage  intermediate  between  the  highest 
apes  and  the  Neanderthal  man. 

Concerning  the  ancestry  of  Pithecanthropus  and  its  rela- 
tionships to  the  apes  the  widest  opinions  still  prevail,  but  the 


44  HISTORY   OF   THE    HUMAN    BODY 

trend  of  opinion  leads  to  the  rejection  of  the  four  living  an- 
thropoids (gorilla,  chimpanzee,  orang  and  gibbon)  as  direct 
ancestral  forms.  Owing  to  the  modifications  time  is  apt  to 
produce  in  animal  species  it  seems  more  logical  to  expect  to 
find  the  connection  in  some  extinct  type,  as,  for  example,  the 
European  Dryopithecus  of  the  middle  Miocene.  As  the  case 
stands  at  present,  however,  there  are  few  animal  species  con- 
cerning which  so  many  of  the  intermediate  links  have  been 
preserved  as  in  the  case  of  man,  and  to  the  scientist  the 
"  missing-links,"  the  discovery  of  which  would  be  of  the 
greatest  importance,  are  not  those  representing  intermediate 
anthropoidal  forms,  but  those  lying  in  the  far  greater  gaps 
lower  down,  as,  for  example,  between  lemurs  and  primitive 
insectivores,  or  between  the  Pantotheria  and  the  theromorphs, 
which  would  throw  further  light  upon  the  reptilio-amphibian 
ancestry  of  the  Mammalia. 

Naturally  the  phylogenetic  stages  which  lie  in  the  direct 
line  of  human  ancestry  are  of  the  most  value  as  historical 
records,  and  as  such  form  the  main  subject  of  study  for  the 
morphologist,  but  collateral  lines  furnish  many  helpful  sug- 
gestions, and  in  cases  where  a  group  of  animals  which  repre- 
sents an  ancestral  line  has  become  wholly  extinct,  dependence 
must  be  placed  upon  the  nearest  related  group,  although  not 
directly  in  the  line  of  descent.  With  this  in  mind  it  will  be 
seen  from  the  foregoing  that  the  phylogenetic  stages  of  the 
greatest  value  in  the  present  discussion  are  the  following : 

1.  Amphioxus. 

2.  Cylostomes. 

3.  Selachians. 

4.  Ganoids. 

5.  Urodeles   (as  a  substitute  for  the   Stegocephali). 

6.  Reptiles   (preferably  the  chelonians,  as  the  nearest  living 

allies  of  the  theromorphs). 

7.  Monotremes  (the  nearest  living  allies  of  the  Pantotheria). 

8.  Marsupials    (probably   not  very   near   the  direct   line,   but 

suggestive    of   the    conditions    in    the    primitive    Insec- 
tivora). 


THE    PHYLOGENESIS    OF   VERTEBRATES        45 

9.  Insectivora    (of    the    modern    type,    still    quite    primitive. 
The  rodents  are  valuable  here  also  as  collateral  lines, 
descended  from  the  primitive  Insectivora). 
10.  Lemurs    (practically   modern  Mesodonta,  and  hence  repre- 
senting fairly  well  the  immediate  ancestors  of  the  anthro- 
poids). 

11.  Cercopithecidoe   (tailed  monkeys  of  the  Old  World). 

12.  The  large  tailless  apes  of  the  Old  World  (Gorilla,  Chim- 

panzee, Orang,  Gibbon). 

13.  Pithecanthropus   (extinct). 

14.  Homo  primigenius  (extinct). 

15.  Homo  sapiens. 

In  this  list  an  attempt  has  been  made  to  enumerate  only 
living  forms,  specimens  that  are  still  available  to  the  anatomist 
for  dissection  and  full  comparison.  In  two  cases,  however, 
13  and  14,  this  resolution  was  broken,  owing  to  the  vital 
importance  of  these  forms.  It  must  also  be  remembered 
that  we  possess  at  least  a  partial  skeletal  record  of  some  of 
the  extinct  groups  that  lie  in  the  direct  line  of  ancestry,  and 
that  these  records,  although  extremely  fragmentary,  are  of  the 
utmost  value.  It  will  also  be  seen  that  these  stages  are  not 
those  of  coordinate  groups,  but  that  they  grade  from  Classes 
to  Orders,  then  to  Families,  and  finally  to  Genera  and  Species ; 
this  is,  however,  the  natural  manner  of  considering  an  an- 
cestry, for  the  early  stages  are  the  less  detailed  and  are  ex- 
pressed equally  well  by  all  the  members  of  a  large  group, 
while  the  finishing  touches,  which  separate  genera  from  ge- 
nera and  species  from  species,  consist  of  slight  differences, 
more  recent  and  superficial  in  character. 

A  similar  gradation  is  seen  in  the  developmental  history, 
as  studied  in  comparative  embryology,  in  which  the  earliest 
features  laid  down  are  those  of  the  main  subdivisions;  then 
come  in  succession  those  of  the  Class,  the  Order,  the  Family, 
and  so  on  until  the  distinguishing  characters  of  the  Species 
make  their  appearance,  the  latter  usually  not  fully  expressed 
until  maturity. 

The  truth  of  this  actual  recapitulation  of  the  history  became 


46  HISTORY    OF   THE    HUMAN    BODY 

apparent  to  the  early  morphologists,  one  of  the  greatest  of 
whom  thus  expressed  his  feelings  while  gradually  tracing  back 
from  the  adult  condition  the  developmental  history  of  the 
skull  of  the  common  fowl :  "  Whilst  at  work  I  seemed  to 
myself  to  have  been  endeavoring  to  decipher  a  palimpsest,  and 
not  one  erased  and  written  upon  again  just  once,  but  five  or 
six  times  over.  Having  erased,  as  it  were,  the  characters  of 
the  culminating  type, — that  of  the  gaudy  Indian  bird, — I 
seemed  to  be  among  the  sombre  Grouse;  and  then,  towards 
the  end  of  incubation,  the  characters  of  the  Sand-grouse  and 
Hemipod  stood  out  before  me.  Rubbing  these  away,  in  my 
downward  work,  the  form  of  the  Tinamou  looked  me  in  the 
face;  then  the  aberrant  Ostrich  seemed  to  be  described  in 
large  archaic  characters;  a  little  while,  and  these  faded  into 
what  could  just  be  read  off  as  pertaining  to  the  sea-turtle ; 
whilst,  underlying  the  whole,  the  Fish  in  its  simplest  Myxinoid 
form  could  be  traced  in  morphological  hieroglyphics."  * 

In  following  out  the  historical  development  of  the  different 
systems,  as  outline^  in  the  ensuing  chapters,  both  embryonic 
and  phylogenetic  records  have  been  drawn  upon  as  the  primary 
sources  from  which  this  history  may  be  deduced,  and  the 
conclusions  which  have  the  corroboration  of  both  may  be 
naturally  considered  the  most  trustworthy  ones.  Each  of  these 
two  records  has  its  advantages  and  its  disadvantages ;  in  the 
former  the  stages  are  continuous,  although  the  early  ones  are 
obscure,  and  all  parts  of  the  record  are  apt  to  be  overlaid  and 
mystified  by  caenogenetic  changes ;  in  the  latter  the  record  is  far 
1  more  fragmentary  and  its  stages  are  discontinuous,  but  the 
facts  are  usually  plainer  and  more  easily  read.  An  adult  lower 
animal  which  represents  a  phylogenetic  stage  in  the  history  of 
a  higher  shows  the  parts  in  full  physiological  efficiency,  while 
in  an  embryonic  stage  the  organs  are  at  the  best  not  wholly 
functional,  and  often  render  it  difficult  to  imagine  an  adult 
animal  with  the  same  relationship  of  parts ;  on  the  other  hand, 
in  places  where  a  long  historic  period  has  no  known  living  or 

*  W.  K.  Parker,  in  Trans.  Roy.  Philos.  Soc ,  1869,  pp.  803-804. 


THE    PHYLOGENESIS    OF   VERTEBRATES        47 

fossil  representative  among  adult  animals,  the  only  clew  is 
that  furnished  by  embryology.  It  will  be  seen  that  in  a  few 
cases,  notably  in  the  history  of  the  transition  from  fins  to 
walking  limbs  between  fishes  and  amphibians,  both  records  are 
unsatisfactory,  and  in  such  cases  the  only  hope  of  a  definite 
solution  lies  in  the  future  discovery  of  some  extinct  form  which 
may  bridge  the  gap  and  thus  furnish  a  clew  by  which  the  two 
discontinuous  threads  may  be  united. 


CHAPTER  III 
THE  ONTOGENESIS  OF  VERTEBRATES 

"...  the  embryological  record,  as  it  is  usually 
presented  to  us,  is  both  imperfect  and  misleading.  It 
may  be  compared  to  an  ancient  manuscript,  with  many 
of  the  sheets  lost,  others  displaced,  and  with  spurious 
passages  interpolated  by  a  later  hand.  .  .  .  Like 
the  scholar  with  his  manuscript,  the  embryologist  has 
by  a  process  of  careful  and  critical  examination  to 
determine  where  the  gaps  are  present,  to  detect  the 
later  insertions,  and  to  place  in  order  what  has  been 
misplaced." 

FRANCIS    BALFOUR,    Comparative    Embryology. 

Vol.  I,  p.  3. 

WITH  the  exception  of  a  few  cases  of  asexual  reproduction, 
that  is,  cases  in  which  an  individual  arises  from  a  single  parent, 
every  multicellular  organism  results  from  a  conjugation  be- 
tween a  macro-  and  a  micro-gamete.  These  are  called  the 
ovum  and  spermatozoon,  respectively,  and  are  the  product  of 
two  distinct  parent  individuals.  Precisely  the  same  phenome- 
non occurs  frequently  among  colonial  unicellular  organisms, 
where  an  entire  colony  produces  gametes  of  only  one  sort, 
and  in  this  case  the  distinction  between  such  a  colony  and 
the  mass  of  cells  which  constitute  the  body  of  a  simple 
Metazoan  is  extremely  slight  and  depends  solely  upon  the 
amount  of  differentiation  between  the  individual  cells  and  the 
consequent  degree  of  mutual  interdependence  attained.  In 
both  cases  the  cell  mass,  aside  from  the  gametes,  constitutes  a 
soma,  composed  in  the  one  case  of  homogenous,  in  the  other 
of  heterogenous,  cells.  The  soma,  or  cell  colony,  is  perishable 
and  restricted  to  a  definite  time  of  existence;  the  gametes  by 
their  conjugation  produce  zygotes,  each  of  which,  by  its  re- 
peated division,  may  form  a  new  soma,  that  is,  the  colony,  or 
the  individual,  of  the  succeeding  generation. 

48 


THE    ONTOGENESIS    OF   VERTEBRATES         49 

Among  multicellular  organisms,  the  gametes  are  produced 
in  definite  organs,  the  gonads,  or  germ-glands,  which  pro- 
duce but  a  single  sort  of  gamete,  either  macro-gametes  (ova) 
or  micro-gametes  (spermatozoa).  Those  glands  are  termed 
respectively  ovaries  and  testes,  and  may  occur  in  the  same  or 
in  different  individuals.  In  the  latter  case  the  individuals 
are  said  to  be  of  separate  sexes  and  are  termed  male  and  fe- 
male, the  former  secreting  the  micro-,  the  latter  the  macro- 
gametes.  Individuals  possessing  both  sorts  of  gonads  are 
termed  hermaphroditic  or  bisexual,  but  owing  to  the  fact 
that  usually  the  two  sorts  of  organs  are  functionally  active 
at  different  times,  the  organisms  are  seldom  functionally  bi- 
sexual, but  alternately  male  and  female.  Such  hermaphroditic 
forms  are  frequent  among  invertebrates,  and  occur  regularly 
in  certain  classes,  but  in  vertebrates  they  are  found  only 
among  the  Cyclostomes  (Myxinoids-),  although  in  all  cases  the 
curious  homology  between  the  parts  in  the  two  sexes  (Cf. 
Chap.  VIII)  suggests  that  the  phenomenon  may  have  been 
widespread  or  even  universal  among  the  ancestors  of  modern 
vertebrates. 

In  most  aquatic  animals  the  gametes  are  liberated  in  the 
water  and  conjugation  takes  place  without  any  act  on  the 
part  of  the  parents,  through  the  motor  action  of  the  micro- 
gametes  themselves,  exactly  as  in  Protozoa;  in  terrestrial 
forms,  however,  since  the  gametes  need  a  liquid  medium,  this 
latter  is  supplied  by  glands,  and  the  seminal  fluid  of  the  male, 
in  which  the  micro-gametes  swim  actively,  is  conveyed  to  the 
female  by  some  form  of  copulation. 

Since  the  superficial  phenomena  are  so  obvious  that  they  are 
universally  recognized  without  technical  study,  while  the  es- 
sential details  require  for  their  detection  the  care  and  patience 
of  an  experienced  microscopist,  and  since  especially  the 
parallel  phenomena  occurring  among  the  Protozoa  have  re- 
mained unknown  until  within  comparatively  recent  years,  it 
may  be  easily  comprehended  that  the  terms  in  common  use 
relative  to  these  phenomena  fail  to  express  the  underlying  bio- 
logical principles  and  are  not  of  universal  applicability.  Thus 


HISTORY   OF   THE   HUMAN    BODY 


the  micro-gametes,  first  discovered  in  the  seminal  fluid  of 
mammals,*  were  termed  spermato-zoa,  or  sperm  animals,  a 
term  expressing  the  view  held  at  that  time  that  they  were 
parasitic  or  adventitious  organisms  occurring  in  a  fertilizing 
or  quickening  fluid,  and  from  this  the  act  of  mixing  the 
spermatic  fluid  with  the  ova  was  termed  fertilization.  This  term 
is  now  applied  technically  to  the  entrance  of  the  spermatozoon 


FIG.  9.  Earliest  stages  of  Metazoan  development. 

The  upper  row  represents  the  egg  of  Sycandra,  a  calcareous  sponge  [after  F.  E. 
SCHULZE];  the  lower  row  represents  that  of  the  rabbit  [after  BISCHOFF].  In  the 
rabbit  the  egg  is  surrounded  by  a  thick  capsule,  the  zona  pellucida.  The  egg  of  the 
sponge  is  without  this  and  floats  freely  in  the  water. 

into  the  ovum,  i.  e.,  to  the  union  of  the  two  gametes,  and  is 
thus  synonymous  with  conjugation,  when  applied  to  Metazoa. 
Furthermore,  since,  in  the  majority  of  cases,  the  bulk  of  the 
ovum  so  far  exceeds  that  of  the  spermatozoon  that  the  latter 
appears  to  be  lost  in  the  process,  the  term  ovum,  or  egg,  is  com- 
monly used  to  designate  not  only  the  macro-gamete  (the  un- 
fertilized egg),  but  also  the  double  cell  resulting  from  the 
conjugation  (the  fertilized  egg),  a  use  of  terms  which  neces- 
sitates constant  watchfulness  in  order  to  guard  against  con- 
fusion. Ovum  and  spermatozoon,  the  macro-  and  micro-ga- 

*  Discovered  in  1677  by  Ludwig  Hamm,  a  pupil  of  Leeuwenhoek. 


THE    ONTOGENESIS    OF   VERTEBRATES          51 

metes  respectively  of  a  conjugation,  are  essentially  Protozoa, 
and  thus  the  first  stage  in  the  development  of  multicellular 
animals  is  an  historic  repetition  representing  the  first  and 
simplest  of  organisms.  The  spermatozoon  with  its  motor 
organ  still  retains  its  protozoan  character  even  in  the  highest 
of  the  vertebrates,  but  the  ovum,  loaded  down  with  yolk, 
bears  for  the  most  part  little  resemblance  to  an  active  or- 
ganism. Even  here,  however,  in  certain  sponges  and  hydroid 
polyps,  a  more  primitive  form  of  ovum  is  still  preserved,  for 
it  is  here  amoeboid  in  form  and  possesses  functional  pseudo- 
podia,  being  often  impossible  to  distinguish  from  genuine 
Amoebae,  the  simplest  of  Protozoa.  This  is  a  good  illustration 
of  Rule  V  of  ontogenesis  as  given  in  the  previous  chapter,  since 
it  is  to  be  expected  that  here,  among  the  lowest  and  simplest 
of  the  Metazoa,  the  early  stages  would  receive  the  fullest 
attention  in  the  ontogenetic  recapitulation. 

In  size  ova  vary  greatly,  but  the  difference  is  due  mainly 
to  the  actual  amount  of  food  stuff,  or  yolk,  which  is  required 
in  each  case:  this  in  turn  is  proportional,  not  to  the  size  of 
the  adult  animal,  but  to  the  degree  of  maturity  at  which  it  is 
most  advantageous  for  the  young  animal  to  begin  its  free 
existence.  Some  animals  produce  a  few  very  large  eggs  and 
thus  use  up  their  reproductive  energy  in  developing  yolk; 
others  produce  large  quantities  of  tiny  eggs  which  will  develop 
into  innumerable  minute  larvae.  Both  extremes  and  all  in- 
termediate grades  are  the  result  of  adaptation  to  the  various 
conditions  that  surround  the  different  organisms  and  thus 
regulate  the  size  of  the  egg,  as  well  as  the  size  and  shape  of 
the  parts  in  the  adult.  Thus,  for  example,  the  ova  of  jelly- 
fish, earth-worms,  many  molluscs,  star-fish,  and  most  mam- 
mals, are  very  small,  almost  microscopic;  those  of  insects, 
crustaceans  and  fishes  are  usually  of  an  appreciable  size,  those 
of  frogs  and  of  certain  fish  are  still  larger,  while  the  eggs  of 
reptiles  and  birds  are  enormous,  those  of  the  latter  having 
reached  the  extreme  limit  relative  to  the  size  of  the  parent. 

In  the  eggs  of  placental  mammals,  which  are  practically 
yolkless,  there  is  no  great  difference  in  actual  size  between 


52  HISTORY   OF   THE    HUMAN    BODY 

such  extremes  as  those  of  the  elephant  and  the  mouse;  in 
the  birds,  on  the  other  hand,  the  true  egg,  i.  e.,  the  yellow 
sphere  usually  termed  the  "  yolk,"  is  approximately  propor- 
tionate to  the  size  of  the  parent.  This  difference  is  due  to 
the  fact  that  in  mammals  the  egg  is  little  more  than  the  first 
cell  of  the  new  individual,  since  the  food  supply  comes  en- 
tirely from  outside  sources,  while  in  birds  the  food  is  placed 
wholly  within  the  egg  and  is  the  only  source  available  to  the 
young  bird. 

The  spermatozoon,  never  having  yolk  to  give  it  bulk,  is  al- 
ways small,  usually  far  beyond  the  limits  of  the  unaided  eye. 
Its  form  is  typically  that  of  an  oval  cell-body  or  "  head  " 
to  which  is  attached  a  locomotive  flagellum,  which  may  at- 
tain an  appreciable  dimension  in  respect  to  length,  but  is  al- 
ways extremely  delicate. 

When  the  seminal  fluid  and  the  ova  are  brought  together 
there  is  always  a  vast  excess  of  spermatozoa,  and  in  cases  in 
which  direct  observation  has  been  possible,  as  in  aquatic  forms, 
in  which  the  mingling  of  the  elements  occurs  freely  in  the 
water,  the  eggs  are  seen  to  be  assailed  by  dozens  of  active 
spermatozoa,  each  endeavoring  to  effect  an  entrance.  To 
permit  the  entrance  of  one  and  only  one  of  the  entire  number, 
several  devices  are  made  use  of  by  the  eggs  of  various  species ; 
one  of  these  is  the  encasement  of  the  entire  ovum  in  a  shell, 
in  which  there  is  a  single  minute  opening,  the  micropyle, 
through  which  a  single  spermatozoon  enters  and  in  so  doing 
effectually  blocks  the  way  for  all  successors.  In  other  cases 
the  entrance  of  a  spermatozoon  seems  to  cause  some  chemical 
or  physical  change  which  renders  the  egg  substance  impervious 
to  the  other  male  cells  or  incompatible  with  their  continued  ex- 
istence. In  the  eggs  of  echinoderms  (star-fish,  sea-urchins, 
etc. )  the  stimulus  of  an  entering  spermatozoon  causes  the  im- 
mediate formation  of  an  external  membrane  which  effectually 
prevents  any  farther  entrance.  In  mammals  it  is  probable 
that  the  zona  radiata  proves  an  impassable  barrier  to  all  sper- 
matozoa except  those  that  approach  it  in  a  direction  perpen- 
dicular to  its  surface,  thus  greatly  reducing  the  number  that 


THE    ONTOGENESIS    OF   VERTEBRATES          53 

are  in  condition  to  enter  the  egg.  It  is  also  likely  that  here, 
as  in  many  other  cases,  several  spermatozoa  may  actually 
enter  the  egg  substance,  but  that  all  except  one  are  simply 
added  to  the  yolk  and  serve  as  food.* 

The  spermatozoon,  after  the  entrance  into  the  egg  is  once 
effected,  drops  its  locomotor  apparatus  and  becomes  merely 
a  nucleus,  which  fuses  with  the  one  belonging  to  the  egg,  a 
procedure  similar  to  conjugation  in  the  Protozoa.  The  egg 
cell  thus  becomes  furnished  with  a  fusion-nucleus,  and  may 
be  considered  from  now  on  the  first  cell  of  a  new  organism. 
From  it  arise  all  the  cells  of  the  developing  animal  through 
the  process  of  fission,  and,  since  a  division  of  the  nucleus  al- 
ways precedes  the  division  of  the  cell,  it  follows  that  this 
fusion-nucleus  is  in  the  same  way  the  primary  one  from  which 
all  later  nuclei  are  to  be  derived.  Since  now,  as  has  been  shown 
bv  direct  observation,  this  fusion-nucleus  becomes  divided 
in  such  a  way  as  to  effect  an  exactly  equal  division  of  both 
maternal  and  paternal  components,  and  since  the  process  has 
been  found  to  continue  as  far  as  the  investigators  have  been 
c.ble  to  follow  it,  it  is  extremely  probable  that  the  nucleus  of 
each  and  every  cell  of  the  adult  organism  contains  an  element 
derived  from  each  of  its  parents. 

Herein  lies  a  material  basis  for  the  phenomena  of  heredity, 
and  it  thus  becomes  evident  that  all  hereditary  traits  and  char- 
acters are  perpetuated  through  the  direct  transmission  and 
growth  of  a  bit  of  material  furnished  by  each  parent  and 
handed  doii'n  to  each  cell  of  the  organism.  Although  this  is 
still  the  mystery  of  mysteries  to  the  biologist,  the  careful  study 
of  the  past  twenty  or  thirty  years,  directed  upon  this  very 
point,  has  revealed  much,  but  in  so  doing  has  added  more 

*  Until  1875  it  was  generally  supposed  that  more  than  one  sperma- 
tozoon took  part  in  the  fertilization  of  an  egg.  The  true  facts  in  the 
case  were  first  determined  by  observation,  and  later  proven  by  direct 
experiment.  Polyspermy,  or  the  introduction  into  the  egg  of  more  than 
one  spermatozoon,  has  been  experimentally  brought  about  in  the  eggs  of 
various  marine  animals  by  such  methods  as  the  application  of  heat  and 
cold  or  the  use  of  poisons,  and  in  all  cases  the  resulting  development  has 
been  abnormal. 


54 


HISTORY   OF    THE    HUMAN    BODY 


that  is  still  unknown.  It  has  shown  the  nucleus  to  be  a  mi- 
crocosm of  extraordinary  complexity,  and  has"  opened  up  a 
new  world,  the  very  existence  of  which  has  until  lately  re- 
mained unsuspected. 

What  seems  to  be  the  essential  element  of  all  nuclei,  found 
alike  in  plants  and  animals,  is  a  substance  which,  from  its 
extreme  susceptibility  to  staining  fluids  when  artifically  treated 
for  purposes  of  microscopic  examination,  has  been  designated 
by  the  non-committal  term  of  chromatin.  During  functional 


VII 


VIII 


FIG.  10.  Diagrams  representing  normal  mitosis. 

In  I  the  nucleus  is  "resting";  the  centrosome  is  seen  by  its  side.  In  II  the 
spireme  appears,  which  in  III  becomes  separated  into  chromosomes.  In  IV  the 
centrosomes  have  become  placed  at  opposite  poles,  while  the  chromosomes  form 
an  equatorial  plate  midway  between  them.  Each  chromosome  divides  longitudinally 
in  V,  and  in  VI  and  VII  becomes  drawn  to  the  two  opposite  poles.  In  VIII  the 
cell  divides  into  two. 

activity  this  substance  is  diffused  throughout  the  nucleus  in 
little,  irregular  masses,  but  assumes  the  form  of  a  continuous 
thread  or  chain  preparatory  to  a  cell  division,  and  eventually 
becomes  separated  into  a  definite  number  of  equal  bodies,  the 
chromosomes.  The  number  of  these  found  in  any  somatic 
cell  of  a  given  species  of  animal  is  always  the  same  for  that 
species,  but  may  be  different  in  an  allied  species,  and  the  num- 
ber seems  to  bear  no  reference  to  the  size  or  the  degree  of 
complexity  of  the  animal.  For  instance,  the  number  four  oc- 
curs in  Ascaris,  the  pin-worm,  eight  in  certain  nematode 


THE    OXTOGEXESIS    OF   VERTEBRATES          55 

worms,  twelve  in  the  mole-cricket,  and  sixteen  in  a  water 
beetle,  the  rat  and  Man,  as  well  as  in  the  pine  and  the 
onion.  Cyclops,  a  minute  crustacean,  possesses  twenty-four, 
as  do  also  the  frog,  mouse,  snail,  lily  and  a  fern  (Osmnnda). 
The  earth-worm  has  thirty-two  chromosomes,  the  torpedo  thir- 
ty-six, and  Artemia,  a  small  shrimp,  the  unusual  number  of 
1 68.  Whenever  a  cell  divides  in  a  growing  or  proliferating 
tissue,  the  maintenance  of  the  same  number  of  chromosomes 
in  each  of  the  two  resulting  cells  is  effected  by  means  of  a 
complex  mechanism  of  minute  threads,  radiating  from  two 
opposite  centers,  which  results  in  the  separation  of  each  in- 
dividual chromosome  into  equal  halves,  thus  assuring  for  each 
daughter  cell,  not  merely  the  same  number  of  chromosomes, 
but  halves  of  the  same  ones.  This  process  is  known  as  mito- 
sis or  karyokinesis. 

To  the  general  rule  concerning  the  constancy  in  the  num- 
ber of  the  chromosomes,  there  is,  however,  one  very  important 
exception,  and  that  is,  in  the  germ  cells,  that  become  the 
gametes  in  a  conjugation,  the  starting  point  of  a  new  organ- 
ism. Here,  owing  to  a  difference  in  their  mode  of  formation, 
the  number  of  chromosomes  in  a  given  species  is  exactly  one- 
half  of  that  characteristic  of  the  somatic  cells  of  the  same 
species,  and  it  is  only  by  the  fusion  of  the  two  gametes,  ovum 
and  spermatozoon,  that  the  normal  somatic  number  is  re- 
stored. This  reduction  of  the  number  of  chromosomes  is 
brought  about  through  an  extremely  complex  process,  the  es- 
sentials of  which  are :  first,  the  formation  of  certain  germ-cells, 
spermatogonium  or  oogonium,  which  develop  twice  the  normal 
number  of  chromosomes,  and,  secondly,  two  successive  divi- 
sions of  the  cells,  and  of  the  number  of  chromosomes  also,  by 
means  of  which  four  cells  are  produced,  each  with  one-half 
the  normal  number.  In  the  case  of  the  male  cells  each  of  the 
four  is  effective,  and,  through  a  metamorphosis  in  its  form, 
becomes  a  functional  spermatozoon;  but  in  the  case  of  the 
female,  owing  to  the  disadvantages  which  would  arise  from 
the  division  of  the  yolk  into  four  ova  of  equal  size,  one  of 
them  retains  it  all  and  becomes  a  functional  ovum  while  the 


56  HISTORY   OF   THE    HUMAN    BODY 

others  become  yolkless,  abortive,  eggs,  attached  to  the  ovum 
and  called  polar  globules.  Owing  to  the  enormous  disparity 
in  size  between  the  abortive  and  the  functional  ova  the  divi- 
sions of  the  oogonium  by  which  they  are  formed  were  for  a 
long  time  not  recognized  as  true  cell  divisions,  but  the  polar 
globules  were  spoken  of  as  extruded  or  cast  off  from  the 


VI  VII 

FIG.  ii.  Diagram  of  fertilization. 

Stage  I  represents  the  egg  just  previous  to  maturation.  The  chromosomes,  ar- 
ranged in  tetrads,  are  twice  the  number  found  in  somatic  cells,  which,  in  this  dia- 
gram is  assumed  to  be  12.  At  II  a  mitotic  figure  is  formed,  which,  in  III,  results 
in  the  formation  of  two  cells;  a  little  one,  the  first  polar  globule,  a,  and  the  egg, 
each  with  a  reduced  number  of  chromosomes,  in  this  case  12.  In  IV  and  V  a 
second  mitotic  figure  is  formed,  which  results  in  the  expulsion  of  a  second  polar 
globule,  b,  and  the  reduction  of  the  chromosomes  of  the  egg  nucleus  to  six,  one- 
half  the  normal  number.  Meanwhile  a  spermatozoon  head  has  entered  the  egg,  com- 
posed mainly  of  chromatin,  the  equivalent  of  the  six  chromosomes  of  the  reduced  egg 
nucleus,  and  a  new  centrosome,  to  replace  that  of  the  egg  which  was  destroyed  during 
the  expulsion  of  the  second  polar  globule.  The  spermatozoon  head  rotates  through 
180°,  thus  bringing  the  centrosome  between  the  male  and  female  germ  nuclei,  as 
in  VI.  The  first  cleavage  spindle  is  seen  forming  in  VII  and  VIII,  after  which 
the  cell  divides  into  two  and  development  begins. 

"egg/'  terms  which  express  the  phenomena  as  observed,  but 
mask  their  true  biological  significance. 

Usually,  owing  probably  to  the  rudimentary  condition  of 
the  polar  globules  and  their  lack  of  function,  the  globule 
formed  by  the  first  of  the  two  reductive  divisions,  and  hence 
the  equivalent  of  the  definite  ovum  plus  one  abortive  egg, 
does  not  carry  through  its  division  into  two,  but  remains  as 


THE    ONTOGENESIS    OF   VERTEBRATES          57 

a  single  mass,  and  is  spoken  of  as  the  "  first  polar  globule  " 
in  distinction  from  that  resulting  from  the  second  division, 
which  is  termed  the  "  second."  Strictly  speaking,  the  first 
and  second  polar  globules  are  not  equivalent,  but  the  first  is 
the  equivalent  of  two  abortive  eggs  and  the  second  of  but 
one ;  and  corresponding  to  this  the  first  polar  globule  pos- 
sesses twice  the  number  of  chromosomes  exhibited  by 
either  the  second  globule  or  the  functional  egg.  Furthermore, 
the  two  polar  globules  are  frequently  not  extruded  until  after 
the  entrance  of  the  spermatozoon,  the  presence  of  which  seems 
to  act  as  a  stimulus  for  these  cell  divisions.  In  these  cases, 
the  unfertilized  "  egg "  is,  strictly  speaking,  not  the  ovum, 
but  the  oogonium,  which  requires  the  two  reductive  divisions 
to  become  the  equivalent  of  the  spermatozoon. 

To  illustrate  this  by  an  actual  example,  let  us  suppose  an 
animal  that  possesses  in  each  somatic  cell  sixteen  chromosomes. 
The  spermatogonium  twould  thus  possess  thirty-two  which,  by 
the  reductive  divisions,  would  result  in  the  formation  of  four 
spermatozoa,  each  with  eight.  Similarly  the  oogonium  would 
possess  thirty-two  chromosomes,  a  number  which  would  be 
reduced  to  sixteen  by  the  expulsion  of  the  first  polar  globule, 
the  latter  body  having  the  like  number.  The  second  reductive 
division  would  result  in  the  formation  of  a  second  polar  body 
with  eight  chromosomes,  and  would  leave  eight  in  the  egg. 
This  number,  when  added  to  the  same  amount  introduced  by 
the  spermatozoon,  restores  the  normal  number,  sixteen,  and 
thus  forms  the  first  cell  of  the  new  organism,  equipped  with 
the  regular  somatic  number,  one-half  from  either  parent. 

In  this  is  seen  a  provision  for  avoiding  that  enormous  in- 
crease in  the  number  of  chromosomes  that  otherwise  must  be 
the  inevitable  result  of  each  conjugation.  Furthermore,  when 
taken  in  connection  with  the  fundamental  law  of  heredity  that 
in  the  long  run  the  two  parents  are  equally  potent  in  trans- 
mitting their  characteristics  to  their  offspring  and  that  neither 
sex  has  the  preponderance  of  influence  in  this  direction,  it  is 
seen  that  the  hereditary  substance  must  lie  in  the  chromosomes 
alone,  since  these  are  the  only  elements  in  which  both  parents 


58  HISTORY    OF   THE    HUMAN    BODY 

are  equally  represented.  Neither  the  preponderating  bulk  of 
the  ovum  (macro-gamete)  nor  the  flagellum  and  other  loco- 
motor  apparatus  of  the  spermatozoon  (micro-gamete)  are 
of  any  significance  in  hereditary  transmission,  but  are  mere 
adaptive  characters,  of  provisional  functional  importance,  and 
without  influence  in  directing  the  development  of  the  new  or- 
ganism ;  while  the  chromosomes,  to  effect  the  union  and  equal 
division  of  which  the  other  parts  have  been  developed,  form 
the  true  germ-plasm,  transmitted  in  direct  continuity  from  both 
parents  and  entering  every  cell  as  it  develops,  directing  both 
the  architectural  plan  which  these  cells  assume  and  also  their 
gradual  differentiation  into  the  tissues  which  form  the  adult 
soma  of  the  succeeding  generation. 

In  this  "  Continuity  of  the  germ-plasm  "  is  found  the  ma- 
terial basis  also  for  the  recapitulation  theory,  the  law  of 
biogenesis  explained  in  the  first  chapter;  for  the  continuously 
living  chroniatin,  which  pervades  each  cell  of  an  organism, 
has  in  its  ozvn  existence  actually  experienced  all  the  somatic 
modifications  of  its  entire  past  history,  traces  of  which  it 
must  retain  in  some  form  of  structural  expression,  enabling  it 
to  control  the  development  of  the  soma  during  every  stage  of 
its  existence.  How  this  is  effected  is  far  beyond  our  present 
means  of  observation,  and  perhaps  of  experiment,  but  the  re- 
sults presuppose  an  inconceivably  complex  structure  in  the 
chromatin  in  order  to  render  such  results  possible. 

The  first  stage  in  the  development  of  all  Metazoa,  that  of 
the  fertilized  ovum  or  zygote,  is  followed,  in  most  cases  imme- 
diately after  fertilization,  by  a  succession  of  cell-divisions,  or 
cleavages,  as  they  are  here  termed,  which,  in  typical  cases,  fol- 
low a  general  geometrical  plan  and  result  in  the  formation  of  a 
mass  of  cells  that  shape  themselves  into  a  definite  embryological 
stage,  that  of  the  blastula.  As  the  various  geometrical  forms 
assumed  by  the  cells  during  the  cleavage  stages  are  all  rep- 
resented among  colonial  one-celled  organisms,  so  there  are 
also  a  few  such  that,  in  the  arrangement  of  their  cells,  closely 
resemble  the  blastula.  In  this  stage  the  cells  form  a  hollow 
sphere,  one  cell  in  thickness,  and  in  cases  in  which  the  blastula 


THE    ONTOGENESIS    OF   VERTEBRATES 


59 


floats  freely  in  the  water,  as  in  that  of  many  of  the  inverte- 
brates, each  cell  is  provided  with  long  vibratile  flagella,  by 
which  the  colony  is  moved.  This  larval  form  is  closely  imi- 
tated by  such  an  organism  as  Volvox,  which  is  usually  reck- 
oned as  a  plant,  but  serves  to  show  a  physiologically  functional 
adult  organism  in  the  corresponding  stage.  The  folding  in, 
or  collapse  of  one  portion  of  the  blastula,  as  in  the  diagram, 


in 


vm 


FIG.  12.  Early  Metazoan  development;  typical.  [After  models  of  Am- 
phioxus  by  HATSCHEK.] 

I,  the  egg.  II,  III,  and  IV,  cleavage  stages.  V  and  VI,  blastula f  in  VI,  which 
represents  a  somewhat  older  stage  than  V;  one-half  has  been  removed.  VII  repre- 
sents the  beginning  of  the  gastrular  invagation,  and  VIII  is  the  completed  gastrula, 
both  sectioned  as  in  VI. 

produces  a  two-layered  cup  which  forms  the  next  important 
ontogenetic  stage,  the  gastrula,  and  in  attaining  this  the 
embryo  passes  beyond  the  Protozoa  in  its  imitative  repetition 
and  assumes  the  essential  form  of  the  simplest  of  the  Metazoa, 
the  Ccelenterata.  A  typical  gastrula  is  radiate  in  structure,  and 
possesses  a  central  axis  with  two  poles,  oral  and  apical,  the 
former  characterized  by  the  presence  of  the  gastrula  mouth 
or  protostome.  This  latter  leads  into  the  large  central  cavity, 
the  gastroccele,  which  has  developed  from  the  exterior  at  the 
expense  of  the  cavity  of  the  blastula,  the  blastoccele.  In  some 


60  HISTORY   OF   THE   HUMAN    BODY 

gastrulae  this  latter  cavity  is  completely  obliterated  by  the  com- 
pletion of  the  process  of  imagination,  but  often  remains  as  a 
space  between  the  two  layers,  the  ectoderm  and  endoderm. 

When  this  type  is  completed  and  becomes  an  adult  animal 
it  often  assumes  a  considerable  complexity  of  structure  but 
never  gets  far  away  from  the  original  plan  and  does  not  de- 
velop more  than  the  two  primary  layers.  The  fresh-water 
hydra  is  an  example  of  one  of  the  simplest  coelenterates  or 
gastrula-animals,  and  the  coral  polyps  and  medusse  represent 
the  more  complex  ones.  In  none  of  these  does  a  blastocoele 
appear,  in  the  simpler  forms  ectoderm  and  endoderm  are 
everywhere  in  contact,  and  in  the  more  complex  medusse  the 
space  between  them  is  filled  by  a  gelatinous  tissue  developed 
from  the  other  layers,  and  termed  mesenchyme* 

Up  to  this  point  the  course  of  development  is  the  same  for 
all  Metazoa,  allowing  for  the  adaptive  modifications  always 
met  with  in  the  application  of  a  general  plan  to  a  group  of 
organisms. 

From  this  point  on,  however,  there  is  a  divergence  in 
the  course  of  development,  and  the  various  branches  of  the 
higher  Metazoa  proceed  along  different  paths,  yet  all  de- 
velop, although  through  different  means,  the  three  following 
attributes,  which  differentiate  them  from  the  lower  Metazoa, 
the  Ccelenterata : 

1.  The  formation  of  a  third  germ  element,  the  mesodenn, 
situated  between  ectoderm  and  endoderm. 

2.  The  formation  of  a  new  cavity  or  system  of  cavities,  the 
metaccele,  lined  wholly  by  the  mesoderm. 

3.  The  attainment  of  a  new  body  axis,  and  a  bilateral,  in- 
stead of  a  radiate,  symmetry. 

Omitting  all  further  reference  to  the  other  branches,  it  ap- 
pears that  in  the  branch  leading  to  the  vertebrates  the  gas- 

*  This  is  carefully  to  be  distinguished  from  the  mesoderm,  or  middle 
layer,  which  appears  first  in  animals  above  the  coelenterates  and  is  always 
in  the  form  of  a  definite  layer.  The  mesenchyme  never  appears  as  a 
layer,  but  its  cells  serve  to  fill  in  the  spaces  between  the  true  germ  layers, 
and  the  structures  formed  from  this  source  are  thus  determined  by  the 
form  of  the  surrounding  tissues. 


THE    ONTOGENESIS    OF   VERTEBRATES          61 

trula  assumes  the  form  and  position  shown  in  Fig.  13,  b,  in 
which  it  becomes  placed  horizontally,  with  the  apical  pole  for- 


a 


•ZtHHHHSSS 


ant, 


FIG.  13.  Diagrams  of  gastrulae.  [Based  on  models  of  Amphioxus  by 
HATSCHEK.] 

(a)  Typical  gastrula,  as  in  Fig.  12,  VIII,  but  differently  placed,  for  comparison 
with  the  others.  (b)  Early  gastrula  of  Amphioxus,  a  probable  ancestor  of  the 
vertebrates,  (c)  Later  embryo  of  the  same. 

xy,  primary  axis,  t.  e.,  that  of  the  gastrula;  ab,  secondary  axis,  that  of  the  adult 
Amphioxus;  en,  neural  canal;  cne,  neurenteric  canal;  np,  neuropore;  ntc,  notochord. 

ward  and  the  protostome  posterior  and  dorsal  and  in  the 
median  line.     The  plan  of  structure  is  a  bilateral  one,  with 


62  HISTORY    OF   THE    HUMAN    BODY 

dorsal,  ventral  and  two  lateral  aspects.  If  this  metamorphosis 
has  any  biogenetic  value,  that  is,  if  it  is  indicative  of  a  genu- 
ine historic  stage  in  the  phylogeny  of  vertebrates,  it  suggests 
an  ancestral  gastrula  that  sank  to  the  bottom,  lay  upon  its 
side  and  exchanged  a  free  swimming  for  a  crawling  mode  of 
locomotion,  apical  pole  forward.  Such  an  hypothetical  form 
as  this  corresponds,  however,  to  nothing  known  at  the  present 
time,  but  may  well  have  disappeared  without  trace,  since  a 
similar  fate  has  happened  to  the  transition  forms  linking 
the  vertebrates  to  the  other  Metazoa,  leaving  the  group  un- 
usually isolated.  [See  Chapter  XII. ] 

There  now  occur  several  simultaneous  changes  which  inau- 
gurate the  essential  vertebrate  structure  and  are  best  explained 
by  the  help  of  the  accompanying  diagrams.  [Plates  I.  and 
II.]  The  gastrula  has  now  become  considerably  elongated  in 
the  direction  of  the  newly  acquired  secondary  axis  and  is  rep- 
resented as  cut  transversely  across,  the  diagram  representing 
the  posterior  portion  and  showing  the  cross-section  as  well  as 
a  portion  of  the  length.  The  most  superficial  of  these  changes 
involves  a  longitudinal  mid-dorsal  stripe,  which  becomes  grad- 
ually turned  in,  forming  a  trough.  Through  the  fusion  in  the 
median  line  of  the  edges  of  the  trough,  the  turned-in  portion 
becomes  a  tube,  which  ultimately  frees  itself  from  its  attach- 
ment to  the  rest  of  the  ectoderm,  and  forms  the  neural  tube, 
the  anlage*  of  the  nervous  system.  The  walls  of  this  tube,  by 
an  excessive  thickening  of  certain  definite  portions,  become  the 
brain  and  spinal  cord,  and  the  lumen  is  perpetuated  as  the  ven- 
tricles of  the  brain  and  the  canalis  centralis  of  the  cord,  the 
embryonal  communication  between  these  cavities  being  re- 
tained throughout  life. 

A  somewhat  similar  structure,  also  median,  arises  from  the 
dorsal  wall  of  the  endoderm.  This  appears  at  first  as  an  in- 
verted trough,  and  possesses  a  narrow  lumen,  but  it  eventually 

*  The  word  Anlage  is  borrowed  from  the  German  to  express  a  concep- 
tion for  which  there  is  no  English  equivalent.  It  signifies  the  first  visible 
indication  of  a  part  that  appears  in  the  embryo,  and  may  thus  signify 
either  a  definite  cell-mass  or  a  slight  change  in  the  arrangement  of  cells. 


PLATE  I.  Diagrams  showing  Vertebrate  development,  ex- 
plained in  the  text;  stages  I  and  II.  Based  upon  a  stereogram  by 
KINGSLEY. 


PLATE    II.      Diagrams     showing     Vertebrate   development; 
stages  III  and  IV.     Based  upon  a  stereogram  by  KINGSLEY. 


THE    ONTOGENESIS    OF   VERTEBRATES          63 

becomes  pinched  off  from  its  place  of  origin,  not  as  a  tube, 
but  as  a  solid  rod  of  cells,  the  notochord,  which  forms  the 
precursor  of  the  vertebral  column. 

A  third  procedure,  more  complicated  than  the  other  two,  is 
that  involved  in  the  formation  of  the  mesoderm.  Like  the 
notochord,  this  arises  also  from  the  endoderm,  and  appears 
typically  in  the  form  of  paired,  lateral  pockets,  the  mesodermic 
diverticula.  There  is  reason  to  suppose  that  originally,  that 
is,  in  certain  of  the  lost  forms  between  the  creeping  gastrula 
and  Amphioxus,  these  diverticula  were  used  as  gonads,  or 
sac-like  cavities,  in  the  lining  of  which  the  germ  cells  were 
developed,  but  in  the  vertebrates  this  function  is  retained  by 
but  a  very  small  portion  of  their  surface,  as  will  be  shown 
later.  These  diverticula  soon  separate  themselves  from  the 
intestinefand  expand  until  they  fill  practically  the  entire  space 
between  ectoderm  and  endoderm  and  lie  in  close  contact  to 
one  another.  They  thus  form  a  series  of  paired  cavities,  the 
metacccles,  those  of  each  side  separated  by  transverse  par- 
titions composed  of  the  walls  of  adjacent  diverticula,  and 
those  of  the  two  lateral  series  similarly  separated  by  longi- 
tudinal partitions  which  lie  in  the  median  line  above  and  be- 
low the  intestine.  The  early  loss  of  the  transverse  partitions 
converts  the  segmental  series  of  lateral  cavities  into  a  single 
pair,  one  for  each  side,  while  a  similar  reduction  of  the  greater 
portion  of  the  ventral  longitudinal  partition  throws  the  two 
cavities  together  and  forms  eventually  a  single  large  metacoele 
or  body  cavity,  lined  by  the  mesoderm.  One  layer  of  this 
invests  the  outer  body  wall,  the  other  the  intestine,  the  parie- 
tal and  'visceral  layers  respectively.  The  longitudinal  parti- 
tions, both  dorsal  and  ventral,  serve  as  suspensory  ligaments 
in  the  intestine  and  are  termed  mesenteries.  The  dorsal  one 
is  retained  throughout  its  entire  extent;  the  ventral  one  disap- 
pears posterior  to  the  liver.  It  will  be  noticed  that  the  meso- 
dermic diverticula  during  their  development  have  expanded 
at  the  expense  of  the  protoccele,  the  original  cavity  included 
between  ectoderm  and  endoderm,  and  thus  at  the  completion 
of  the  process  the  protoccele  has  become  reduced  to  a  com- 


64  HISTORY    OF    THE    HUMAN    BODY 

plicated  system  of  narrow  spaces  lying  everywhere  between 
the  other  layers.  The  protoccele  is  thus  called  the  primary 
and  the  metaccele  the  secondary  body-cavity,  and  it  is  this 
latter,  the  one  lined  by  the  mesoderm  and  included  between  its 
two  layers,  that  forms  the  permanent  body-cavity  of  verte- 


a 


FIG.  14.  Diagrammatic  cross  sections  through  vertebrate  embryos,  based 
upon  the  conditions  found  in  selachians.  [Modified,  after  VAN  WIJHE.] 

(a)  Earlier  stage,  in  which  the  three  parts  of  the  mesodermic  diverticula  are 
still  continuous.  (b)  Later  stage,  in  which  the  epimeres  of  the  mesodermic  di- 
verticula  have  separated  from  the  meso-hypomeres  and  form  a  continuous  layer 
around  the  body,  interrupted  only  at  the  mid-dorsal  and  the  mid-ventral  lines. 

In  all  the  figures  the  ectoderm  is  represented  by  square  cells,  the  endoderm  by 
crossing  diagonal  lines,  the  mesoderm  by  solid  black,  the  mesenchyme  by  dots. 
I,  epimere;  II,  mesomere;  III,  hypomere;  a,  aorta;  g,  gonad;  *,  intestine;  m,  myo- 
tome  of  epimere;  me,  metaceele  (the  definite  crelom) ;  n,  nerve  cord;  nc,  notochord; 
nph,  nephridium;  sk,  sklerotome,  the  anlage  of  the  axial  skeleton;  w,  protonephrotic 
duct  (Wolffian  duct). 

brates,  the  so-called  dcelom  or  pleuro-peritoneal  cavity.  The 
narrowed  spaces  of  the  protoccele  become  filled  with  embry- 
onal connective  tissue  cells,  the  tnesenchyme -,  which  never  as- 
sume the  form  of  a  definite  layer,  and  are  produced  by  pro- 
liferation from  the  mesoderm,  and  perhaps  from  the  other 
two  as  well.  Canals  are  left  here  and  there  which  in  time 
are  built  up  into  a  continuous  system  of  vessels,  with  walls  of 


THE    ONTOGENESIS    OF   VERTEBRATES         65 

connective  tissue,  and  form  the  vascular  system  (blood-vessels 
and  lymphatics). 

The  arrangement  of  the  various  embryonic  elements  at  this 
point  is  shown  in  the  accompanying  diagrams  based  upon 
selachian  embryos,  and  exhibiting  the  actual  proportions  as 
they  exist  in  a  rather  primitive  vertebrate.  [Fig.  14.]  The 
general  arrangement  of  parts  in  an  adult  dog-fish  is  not  ma- 
terially different  from  the  last  of  these.  Through  the  forma- 
tion of  a  restricted  middle  area,  the  mesodermic  diverticula 
become  divided  into  dorsal,  middle  and  ventral  portions,  the 
epimere,  mesomere  and  hypomere  respectively,  each  with  a 
distinct,  separate  history. 

The  epimere,  the  inner  wall  of  which  becomes  greatly  thick- 
ened, eventually  cuts  itself  off  from  the  remaining  meso-hypo- 
mere,  and  expands  both  dorsally  and  ventrally  between  the 
latter  and  the  ectoderm  until  it  meets  the  opposite  one  in  the 
mid-dorsal  and  mid-ventral  lines,  separated  only  by  thin  strips 
of  connective  tissue.  From  the  thickened  inner  wall  of  this 
develop  the  voluntary  muscles  of  the  body,  the  segmentation 
of  which  is  retained  among  the  fishes  throughout  the  greater 
part  of  the  body,  and  still  appears  in  unmistakable  traces 
among  the  highest  forms.  The  mid-ventral  connective  tissue 
partition  separating  the  muscle  masses  of  the  two  sides  be- 
comes the  linea  alba,  a  conspicuous  white  line,  which  persists 
in  all  vertebrates.  The  cavity  of  the  epimere  becomes  sup- 
pressed by  the  growth  of  the  inner  wall  and  thus  comes  to 
nothing. 

The  consecutive  meso-hypomeres  soon  lose  their  independ- 
ence through  the  breaking  down  of  the  transverse  partitions, 
as  described  above,  but  the  metameric  repetition  found  among 
the  parts  derived  from  them  continues  to  suggest  their  origin 
as  separate  diverticula.  From  the  narrowed  mesomere  there 
arise  the  essential  organs  of  the  urogenital  system,  many  parts 
of  which  retain  throughout  life  the  indications  of  a  segmental 
origin.  The  cavities  of  the  mesomere  become  those  of  the 
systems  derived  from  it. 

The  hypomeres,  fused  into  a  single  bag  or  sac,  form  the 


66  HISTORY   OF   THE    HUMAN    BODY 

definite  ccelom,  or  pleuro-peritoneal  cavity,  of  which  they  fur- 
nish the  lining  membrane,  the  peritoneum.  The  outer  layer 
(parietal  mesoderm)  lines  the  body  wall;  the  inner  (visceral 
mesoderm)  invests  the  primary  intestine  and,  later  on,  its 
derivative  organs,  as  lungs,  pancreas  and  liver.  In  all  ex- 
cept mammals  the  membrane  is  a  continuous  one,  but  here, 
through  the  formation  of  the  diaphragm  and  the  consequent 
setting  apart  of  a  separate  thoracic  cavity,  the  portion  thus 
cut  off  is  treated  as  a  distinct  membrane  and  called  the  pleura. 

Although  the  above  sketch  represents  the  underlying  plan 
upon  which  the  development  of  all  vertebrates  is  based,  it  is 
not  found  in  an  unmodified  condition  save  in  the  lowest  classes. 
It  is  most  typically  represented  in  the  development  of  Am- 
phioxus,  for  which  the  foregoing  description,  save  in  a  few 
points,  might  well  be  used;  in  the  selachians,  also,  the  modi- 
fications are  not  very  great  and  the  plan  may  be  easily  traced. 
In  the  amphibians,  however,  the  plan  is  so  much  obscured, 
especially  in  its  earlier  stages,  that  for  a  long  time,  during  the 
early  history  of  the  science  of  embryology,  the  homologies 
were  not  recognized.  These  modifications  become  still  greater 
in  the  Sauropsida  and  Mammalia,  in  which,  without  the  help 
of  the  amphibians,  which  here,  as  elsewhere,  form  a  valuable 
connecting  link,  the  recognition  of  the  early  stages  would  be 
hardly  possible.  The  principal  disturbing  factor,  at  least  in 
amphibians  and  the  sauropsida,  is  the  presence  of  increasingly 
greater  quantities  of  yolk,  which  presents  numerous  mechan- 
ical problems,  and  its  influence  is  felt  with  equal  emphasis  in 
the  case  of  placental  Mammals,  where  the  egg,  although  yolk- 
less,  has  evidently  become  so  through  a  secondary  reduction 
and  still  follows  in  its  development  that  of  the  yolk-filled  eggs 
of  the  Sauropsidan  type. 

One  of  the  most  important  modifications  in  the  develop- 
mental history  of  the  higher  classes  concerns  the  appearance 
and  subsequent  development  of  the  mesoderm  and  the  forma- 
tion of  the  definite  ccelom.  In  Amphioxus  the  pairs  of  di- 
verticula  arise  in  quite  typical  fashion  from  the  sides  of  the 
primitive  intestine,  and  this  procedure  is  almost  as  easily 


THE    ONTOGENESIS    OF   VERTEBRATES 


67 


recognized  in  the  case  of  the  selachians.  The  amphibians  show 
considerable  modification,  and  these  are  the  last  in  the  ascend- 
ing scale  in  which  the  diverticula  are  provided  from  the  first 


FIG.  15.  Four  cross-sections  of  vertebrate  embryos  showing  develop- 
ment of  the  mesoderm. 

(a),  Amphioxus  [after  HATSCHEK];  (&)  Triton  (a  salamander)  [after  HERT- 
WIG];  (c)  bird,  diagrammatic;  (c?)  mole  [after  HEAPE], 

k,  ectoderm;  n,  endoderm;  m,  mesoderm;  mt,  parietal  layer  of  the  mesoderm; 
mp,  visceral  layer  of  the  mesoderm;  v,  nerve  cord;  t,  notochord;  w,  Wolffian  duct; 
g,  gastroccele. 

with  a  definite  lumen,  which  is  here  in  the  form  of  an  irregular 
crack  between  the  outer  and  inner  cell  layers. 

Above  this  class  the  mesoderm  appears  first  in  the  form  of 
an  irregular  cell  layer  which  starts  at  the  sides  of  the  noto- 
chord and  invades  the  space  between  ectoderm  and  endoderm. 


68 


HISTORY   OF    THE    HUMAN    BODY 


In  it  the  region  of  the  epimeres  becomes  easily  distinguished 
by  a  great  increase  in  the  thickness  of  the  layer,  and  an  indi- 
cation of  the  separate  diverticula  appears  through  a  series  of 
transverse  fissures,  which  divide  the  mass  into  separate  square 
blocks,  the  so-called  mesodermic  somites.  These  first  appear 
at  about  the  middle  of  the  body,  and  are  added  to  progres- 


FIG.  16.  Three  early  vertebrate  embryos,  showing  mesodermic  somites. 

(a)  turtle  [after  MITSUKURI].  (&)  chick  [after  DUVAL].  (c)  pig  [after  KEIBEL]. 
nt,  nerve  cord  (brain)  ;  nt',  nerve  cord  (spinal  cord)  ;  ms,  mesodermic  somites;  e,  ear; 
yv,  yolk  veins. 

sively  both  anteriorly  and  posteriorly  until  the  full  number  is 
reached.  The  meso-hypomeric  region  remains  for  a  time  as  a 
single  undivided  layer,  but  ultimately  splits  into  two,  outer  and 
inner,  containing  between  them  a  single  undivided  space,  the 
future  ccelom.  This  latter  is  here  called  a  schizoccele,  in  re- 
spect to  its  mode  of  origin. 

There  is  thus  attained  in  the  higher  vertebrates  a  much 
shortened  and  greatly  modified  method  of  producing  the  ele- 


THE    ONTOGENESIS    OF   VERTEBRATES         69 

ments  for  the  later  developments,  hardly  recognizable  on  com- 
paring it  with  the  more  expanded  and  simple  form  found 
among  the  lower  types.  Aside  from  such  modifications  as 
those  mentioned,  which  are  explained  through  mechanical 
exigencies,  there  appear  to  be  differences  in  the  origin  of  the 
first  mesoderm  cells  themselves,  differences  which  tend  to 
shake  our  faith  in  the  absolute  homology  of  the  germ  layers. 
Since,  however,  in  spite  of  such  variation  in  the  early  history, 
the  same  embryonic  elements  eventually  appear  in  all  cases, 
so  that  the  anlagen  of  the  principal  organs  are  the  same  for 
all,  it  is  hardly  possible  that  the  early  modifications,  however 
profound,  have  any  deeper  significance  than  that  of  caenoge- 
netic  adaptations  to  the  various  changed  conditions  of  develop- 
ment. 

The  presence  of  yolk  has  a  great  modifying  influence,  both 
on  the  general  shape  of  the  early  embryo  and  upon  the  definite- 
ness  of  its  stages.  Yolk  is  an  inert  substance,  the  presence  of 
which  in  large  quantities  within  the  cells  interferes  with  their 
normal  division  and  with  their  assumption  of  normal  positions. 
Beyond  a  certain  proportion,  in  fact,  no  cell  division  is  possi- 
ble, and  the  egg  comes  to  consist  of  two  portions,  (i)  the 
protoplasmic  area,  in  which  all  cell  divisions  take  place,  and 
which  ultimately  becomes  developed  into  the  embryo,  and  (2) 
the  yolk-sac.  These  two  areas  are  indicated  in  some  eggs,  as 
in  those  of  the  frog,  by  a  difference  in  color,  the  protoplasmic 
area  being  deeply  pigmented  and  the  yolk  area  not.  The 
extreme  of  disproportion  is  seen  in  the  bird's  egg,  where  the 
protoplasmic  area  is  represented  by  the  light  yellow  embryonal 
disc,  about  4-5  mm.  in  diameter,  which  floats  on  the  upper 
surface  of  the  huge,  non-cellular  yolk  mass.  In  such  cases, 
the  embryo,  when  passing  through  the  early  stages,  or  until 
after  the  establishment  of  the  mesodermic  somites  and  the 
formation  of  head  and  tail,  is  spread  out  on  the  surface  of  the 
spherical  yolk,  in  proportion  to  which  it  is  so  small  as  to  be 
almost  flat,  but  later  on  becomes  nearly  separated  from  it,  re- 
taining its  connection  by  a  narrow  yolk-stalk  attached  in  the 
umbilical  region.  The  embryo  grows  at  the  expense  of  the 
yolk-sac,  and  as  the  former  increases  in  size,  the  latter  dimin- 


70  HISTORY   OF   THE   HUMAN   BODY 

iS&fc, 


FIG.  17.  Diagrams  of  Amniotes,  showing  the  relation  of  the  extra- 
embryonal  membranes. 

(A)  Sauropsidan,  with  functional  yolk-sac  and  respiratory  allantois.  (B)  Mam- 
ma!, with  functionless  yolk-sac  and  with  the  allantois  converted  into  an  umbilical 
cord  and  placenta. 


THE    ONTOGENESIS    OF   VERTEBRATES         71 

ishes,  so  that  by  the  time  the  animal  assumes  a  free  life  the 
yolk-sac  has  nearly  or  wholly  disappeared. 

The  embryos  of  the  higher  vertebrates  differ  from  those 
of  the  lower  in  one  very  conspicuous  feature,  and  that  is,  in 
the  possession  of  fetal  membranes,  external  to  the  embryo  and 
designed  in  part  for  protection  and  in  part  for  the  obtaining 
of  nourishment.  The  two  membranes  of  the  most  extensive 
occurrence  are  the  amnion  and  the  allantois,  which  are  present 
in  reptiles,  birds  and  mammals  and  absent  in  fishes  and  am- 
phibians, a  difference  which  is  expressed  in  the  two  terms 
Amniota  and  Anamnia  (with  and  without  amnion),  applied 
respectively  to  the  two  divisions  in  question. 

The  amnion  appears  to  be  solely  for  the  protection  of  the 
embryo.  It  is  a  thin  transparent  membrane,  composed  of  parie- 
tal mesoderm  and  ectoderm,  and  is  formed  by  the  growth  of 
folds  about  the  embryo.  It  invests  the  latter  on  all  sides  and 
forms  about  it  an  enclosed  space,  the  amniotic  cavity,  in  which 
the  embryo  lies,  immersed  in  a  colorless  amniotic  fluid,  of 
about  the  same  specific  gravity  as  the  embryo  itself.  The 
allantois  is  in  the  form  of  an  empty  sac,  composed  of  two 
layers,  visceral  mesoderm  and  endoderm,  and  develops  from 
the  umbilical  region  of  the  embryo.  In  reptiles  and  birds  it 
pushes  its  folded  edges  between  yolk-sac  and  amnion  on  the 
inner,  and  the  shell  on  the  outer,  side,  and  thus  comes  to  com- 
pletely invest  the  former  and  line  the  latter  with  a  double  mem- 
brane. In  this  there  develop  two  large  allantoic  (umbilical) 
arteries  and  two  allantoic  veins,  and  the  organ  thus  serves  as 
an  excellent  respiratory  organ,  affecting  the  interchange  of 
gases  through  the  porous  shell.  In  placental  mammals  the 
egg-shell  is  replaced  by  a  membranous  chorion,  and  the  allan- 
tois effects  a  close  union  with  this,  either  involving  the  entire 
surface  or  more  generally  a  restricted  area,  and  this  surface, 
entering  into  a  more  or  less  intimate  relationship  with  the 
mucous  membrane  of  the  maternal  uterus,  forms  the  essential 
organ  of  nutrition,  the  placenta.  That  portion  of  the  chorion 
which  is  involved  in  the  formation  of  a  placenta  is  covered  by 
branching  processes,  the  chorionic  villi,  forming  a  surface 


72  HISTORY   OF   THE    HUMAN    BODY 

known  as  the  chorion  frondosum  in  distinction  from  the  smooth 
area,  the  chorion  Iceve.  A  diffuse  placenta,  where  the  villi 
cover  the  entire  external  surface  of  the  chorion,  is  the  most 
primitive  type,  and  is  found  in  pigs,  horses,  whales  and  por- 
poises; if  a  small  portion  of  the  chorion  is  left  smooth,  the 
placenta  is  bell-shaped,  as  in  some  edentates  and  lemurs.  By 
a  continuation  of  this  process,  that  is,  by  a  farther  extension 
of  the  smooth  area,  the  placenta  becomes  discoidal,  which  is 
the  form  characteristic  of  Man  and  the  higher  anthropoids, 
insectivores,  bats  and  rodents ;  in  the  lower  monkeys  there  are 
two  such  discs,  placed  at  opposite  poles,  the  placenta  discoidea 
duplex.  It  is  the  single  discoidal  type,  as  found  in  man,  that 
gave  the  name  "  placenta  "  to  this  organ,  as  the  word  signi- 
fies a  round,  flat  cake. 

If  there  are  two  smooth  areas  at  opposite  poles,  with  pla- 
cental  villi  between  them,  the  zonary  placenta  is  formed,  the 
type  characteristic  of  all  carnivores,  elephants,  Hyrax  and  the 
Sirenia.  A  very  distinct  type  of  placentation  is  the  cotyle- 
donal,  characteristic  of  ruminants.  Here  the  placental  struc- 
ture is  confined  to  small  nodules  or  cotyledons  scattered  over 
the  entire  surface  of  the  chorion,  and  varying  in  number  from 
three  to  five  in  the  deer  to  more  than  a  hundred  in  the  sheep 
and  cow. 

All  of  the  above  forms  of  placentation  are  easily  derived 
from  the  primitive  diffuse  type,  and  as  a  rule  actually  pass 
through  the  changes  during  early  development,  the  form  finally 
assumed  being  attained  through  the  growth  of  smooth  areas 
(chorion  lave). 

The  methods  of  placentation  may  be  again  divided  with 
reference  to  the  relationship  to  the  uterine  mucous  membrane ; 
in  one  type  the  villi  at  birth  are  simply  drawn  out  of  the  ma- 
ternal portion,  leaving  pits,  and  in  the  other  type  the  union 
between  fetal  and  maternal  elements  is  more  intimate  and  the 
separation  occurs  between  the  mucous  and  muscular  walls  of 
the  uterus  itself,  thus  involving  the  loss  of  maternal  mucous 
membrane,  called  in  this  connection,  the  decidua.  The  latter 
of  these  types,  in  which  the  placenta  becomes  a  far  more  spe- 


THE   ONTOGENESIS    OF   VERTEBRATES         73 

cialized  organ,  is  termed  detiduate,  the  former  indeciduate. 
In  ungulates  and  in  many  of  the  edentates  the  placenta  is  in- 
deciduate, in  most  others  it  is  deciduate. 

When  the  fertilized  egg  of  a  placental  mammal  first  enters 
the  uterus  it  does  not  at  once  become  fixed,  and  development 
proceeds  for  some  time  before  there  is  any  attempt  at  the  for- 
mation of  a  placenta.  Meanwhile  the  egg  passes  through  a 
series  of  typical  cleavage  stages  and  attains  the  condition  of  a 
hollow  sphere,  similar  to  the  blastula  of  more  typical  onto- 
genesis. This,  however,  is  not  a  blastula,  but  the  blastodermic 
vesicle,  upon  one  side  of  which  there  develops  an  embryonal 
area  similar  to  that  of  the  bird,  that  is,  spread  out  in  the  form 
of  a  flattened  disc,  and  not  cylindrical  as  in  the  case  of  other 
yolkless  eggs.  This  apparently  useless  circumlocution  can  be 
understood  only  on  the  ground,  supported  also  by  the  early 
development  in  the  marsupials  and  monotremes,  that  mammals 
have  been  derived  from  ancestors  having  large,  yolk-filled 
eggs  and  that  the  secondary  reduction  of  this  substance  has 
been  too  recent  to  effect  a  corresponding  modification  in  the 
course  of  development.  Adhesion  to  the  walls  of  the  uterus 
occurs  through  the  formation  of  chorionic  villi  over  the  sur- 
face of  the  blastodermic  vesicle,  in  which  the  form  of  placen- 
tation  characteristic  of  the  species  soon  becomes  manifest. 

The  later  developmental  history  of  vertebrates  subsequent 
to  the  formation  of  the  germ  layers  and  the  establishment  of 
the  anlagen  of  the  various  systems,  belongs  to  that  division 
of  the  subject  known  as  organogeny,  or  the  development  of 
the  various  organs,  and  cannot  be  followed '  further  in  this 
place;  it  receives  a  fuller  treatment,  however,  in  the  ensuing 
chapters,  where  the  systems  are  considered  separately  and 
where  embryological  facts  are  made  use  of  in  so  far  as  they 
are  needed  to  explain  the  history  of  the  several  organs.  Most 
of  the  systems  arise  from  a  single  germ  layer,  often,  indeed, 
from  a  definite  restricted  locality  in  one  of  them,  the  anlage  of 
which  appears  at  an  early  period,  and  there  is  thus  a  time  at 
which  an  organ,  however  complex  and  difficult  to  understand 
as  it  exists  in  the  adult,  is  exceedingly  simple.  This  primitive 


74  HISTORY   OF   THE    HUMAN    BODY 

condition  furnishes  the  best  possible  starting  point  from  which 
to  follow  its  gradual  modifications  step  by  step  until  the  adult 
form  is  reached.  The  derivation  and  original  anlage  of  most 
of  the  systems  have  been  given  above  and  are  expressed  graph- 
ically in  several  of  the  diagrams  [Plates  I  and  II;  Fig.  14], 
but  it  may  be  also  useful  to  introduce  the  chapters  on  the 
several  systems  by  a  table  which  shows  the  derivation  of  each. 
In  studying  this  it  must  be  borne  in  mind  that  the  mesenchyme, 
which  is  everywhere  distributed  and  forms  all  of  the  connect- 
ive tissues  of  the  body,  enters  into  the  final  structure  of  every 
other  part,  and  hence  is  not  taken  into  consideration  here. 

EMBRYONAL    ELEMENT.  DERIVATIVE 

I.  Ectoderm Epidermis;  including  that  of  the  entire  exter- 
nal surface,  as  well  as  the  more  external 
parts  of  mouth  cavity,  rectum,  and  other 
cavities  opening  to  the  exterior. 
Epidermic  structures;  including  all  glands  of 
the  integument,  nails  and  claws,  hair  and 
feathers,  horny  scales,  the  enamel  of  the 
teeth  and  the  crystalline  lens. 
Nervous  System;  including  brain  and  cord; 
peripheral  nerves  and  sympathetic  system 
with  the  ganglia  associated  with  each;  the 
epithelium  of  the  sense  organs,  and  the 
tapetum  of  the  eye. 

II.  Endoderm Alimentary  canal,  that  is,  its  essential  layer, 

the  mucous  membrane;  also  all  organs  de- 
rived from  this,  as  thymus  and  thyroid 
glands,  larynx,  trachea  and  lungs,  liver  and 
pancreas. 

Notochord;  the  anlage  about  which  the  ver- 
tebrae    (mesenchymatous     structures)      are 
formed. 
III.  Mesoderm. 

a.  Epimeres Voluntary  muscles,  except  those  of  jaw,  hyoid 

and  branchial  arches. 

b.  Mesomeres. .  Urogenital  system,  including  the  germ  glands. 

c.  Hypomeres. .  Peritoneum;    including    pleura    of    mammals; 

germ-glands;    voluntary    muscles    of    jaw, 


THE   ONTOGENESIS    OF   VERTEBRATES         75 

hyoid  and  branchial  arches,  including  the 
muscles  of  the  larynx. 

IV.  Mesenchyme Connective    tissues;    including    those    in    the 

strict  sense,  also  cartilage  and  bone,  and 
the  corium.  Involuntary  muscles  of  the 
viscera  and  of  the  skin. 

Vascular  system ;  including  heart,  blood-vessels 
and  blood;  lymphatics;  and  the  septum  of 
the  diaphragm. 


CHAPTER    IV 
THE  INTEGUMENT  AND  THE  EXOSKELETON 

"  Seit  Huxley  seine  Schrift  '  Zeugnisse  fiir  die  Stel- 
lung  des  Menschen  in  der  Natur '  veroffentlicht  hat, 
sind  31  Jahre  vergangen,  und  wenn  man  erwagt,  was 
in  diesem  Zeitraum  auf  dem  Gebiet  der  physischen 
Anthropologie,  der  Embryologie  und  Morphologic 
iiberhaupt  gearbeitet  und  erreicht  worden  ist,  so  ist  es, 
meine  ich,  an  der  Zeit,  den  Blick  wieder  einmal  ruck- 
warts  zu  richten,  das  zu  einem  einheitlichen  Ganzen 
zusammenzufassen,  was  an  vielen  Orten  zerstreut 
liegt,  tin  daraus  endlich  zu  ersehen,  was  der  Mensch 
war,  was  er  ist,  und  was  er  sein  wird." 

ROBERT  WIEDERSHEIM,  Der  Bau  des  Menschen, 

1893,  P-  3- 

THE  usual  invertebrate  form  of  integument  is  composed  of 
a  single  layer  of  epidermic  cells,  the  external  surface  of  which 
is  covered  by  a  non-cellular  structure  formed  from  the  cell 
walls.  This  outer  element  is  often  a  transparent  cuticula*  or 
in  other  cases  may  consist  of  vibratile  cilia.  Beneath  the  in- 
tegument, and  separated  from  it  by  a  thin  layer  of  connective 
tissue,  lie  the  muscles. 

The  integument  of  Amphioxus  conforms  to  this  general 
type,  but  in  all  true  vertebrates  important  changes  take  place, 
rendering  it  quite  different  in  structure  and  of  far  greater 
complexity.  The  epidermis  becomes  many-layered  and  loses 
the  external  cuticula,  although  cilia  persist  in  a  few  early 
larval  forms,  and  the  underlying  connective  tissue  becomes 
thick,  often  much  exceeding  the  epidermis  in  this  respect.  As 
this  latter  layer,  the  corium  \_cutis~],  is  almost  indissolubly  as- 

*  The  flattened  outer  cells  of  the  epidermis,  which  form  the  stratum  cor- 
neum,  are,  under  certain  circumstances,  easily  separated  from  the  next, 
and  form  a  thin  layer  often  termed  the  "cuticle."  The  use  of  the  word 
in  this  connection  is  questionable,  on  account  of  its  liability  of  being 
confused  with  the  non-cellular  cuticula  of  invertebrates 

76 


THE    INTEGUMENT   AND    THE    EXOSKELETON       77 

sociated  with  the  epidermis,  while  very  loosely  attached  on 
its  under  side  to  the  parts  which  it  covers,  the  two  form  to- 
gether an  easily  detachable  part,  known  as  the  skin  or  hide, 
similar  in  general  function  to  the  integument  of  invertebrates, 
but  far  more  complex  in  structure.  The  vertebrate  integument 
is  further  characterized  by  a  great  variety  of  secondary  struc- 
tures, involving  one  or  both  layers  and  either  remaining  be- 
neath the  surface,  as  is  the  case  with  glands  and  pigment,  or 
projecting  conspicuously  beyond,  as  in  hairs,  feathers  and 
scales. 

Concerning  the  integument  itself,  in  so  far  as  it  can  be 
treated  apart  from  its  accessory  organs,  it  may  be  noted  that 
the  epidermis  is  always  several  cells  deep  and  is  in  constant 
growth,  being  renewed  from  the  innermost  layer  in  about  the 
same  proportion  as  it  is  worn  off  at  the  surface.  This  inner 
layer  is  a  fairly  definite  one  and  is  termed  the  stratum  gcrm- 
inativiim  [sir.  mucosum  or  Malpighii].  Its  cells  are  con- 
stantly proliferating  and  the  older  cell  generations  are  grad- 
ually pushed  toward  the  surface,  becoming  flattened  and  more 
cornified  as  they  progress.  They  thus  form  a  protective  cov- 
ering for  the  more  delicate  cells  that  lie  beneath  them,  and 
compose  a  layer,  which,  in  distinction  to  the  stratum  germ- 
inativum,  is  called  stratum  cornenm.  Some  authorities  dis- 
tinguish for  convenience  a  stratum  lucidum,  lying  between  the 
two,  although  the  exact  limits  of  none  except  the  stratum 
germinativum  are  definitely  fixed. 

It  is  evident  that,  in  order  to  avoid  an  excessive  growth  of 
these  upper  layers,  there  must  be  some  way  by  which  they 
may  be  continually  removed.  This  is  accomplished  in  reptiles 
and  amphibians  by  periodic  moults  or  ecdyses,  through  which 
the  entire  surface  layer  is  cast  off  by  a  single  process,  and 
quite  often  in  one  continuous  piece,  after  the  formation  of  a 
new  layer  beneath  it.  In  many  forms  with  a  cornified  skin,  like 
snakes  and  lizards,  these  cast-off  "  skins,"  the  exuviat,  are 
matters  of  common  observation,  and  are  seen  to  reproduce 
most  faithfully  every  scale,  horn  or  other  protuberance  charac- 
teristic of  the  animal ;  in  certain  other  cases  the  cast-off  skin 


78  HISTORY   OF   THE    HUMAN    BODY 

is  eaten  by  the  owner.  In  birds  and  mammals  there  is  no 
periodic  moult,  so  far  as  the  skin  is  concerned,  and  no  con- 
tinuous layer  cast  off,  but  the  dead  and  dried  cells  are  con- 
stantly being  worn  from  the  surface  and  pass  away  unnoticed. 
In  these  animals,  however,  there  is  usually  a  definite  period 
for  the  renewal  of  the  accessory  parts,  the  feathers  and  hairs, 
a  form  of  moult  to  be  carefully  distinguished  from  the  fore- 
going. 

The  corium,  in  common  with  other  connective  tissues  and 
in  contrast  to  the  epidermis,  is  not  composed  wholly  of  cells, 
but  consists  in  great  measure  of  fibers,  which  run  in  all  direc- 
tions between  the  cells  and  are  produced  through  their  agency. 
These  fibers,  which,  though  not  the  formative  element  of  the 
corium,  are  the  most  important  structural  ones,  form  a  rather 
loose  and  often  very  elastic  felting,  which,  in  many  vertebrates, 
notably  mammals,  forms  the  main  bulk  of  the  skin.  In  fact, 
it  is  this  layer  alone,  which,  artificially  thickened  by  the  action 
of  tannin,  is  used  for  leather,  the  epidermis  being  first  removed 
by  maceration.  The  corium  is  the  thickest  in  mammals,  but 
is  also  fairly  thick  in  amphibians  and  in  many  fishes.  In  rep- 
tiles and  birds  it  is  thin,  the  amount  of  protection  thus  lost 
being  compensated  for  by  the  dense  and  firm  covering  afforded 
by  the  accessory  epidermic  structure,  scales  and  feathers  re- 
spectively. Birds  have  the  thinnest  corium  of  all  vertebrates, 
a  condition  undoubtedly  correlated  with  the  development  of 
the  feather  coat,  which  renders  the  protection  of  a  thick  corium 
superfluous. 

In  the  formation  of  the  accessory  organs  each  of  the  two 
layers  furnishes  materials  characteristic  of  itself,  and,  although 
in  later  growth  a  structure  that  originates  in  one  layer  can, 
and  generally  does,  invade  the  province  of  the  other,  there  is 
a  definite  place  of  origin  for  each  element  involved.  Thus 
from  the  epidermis  come  intppnm*u.tn.l  gland*  of  all  sorts,  al- 
though they  usually  dip  down  into  the  corium  from  which  they 
receive  a  fibrous  investment.  Pigment  may_ba-.deriveH  from 
cither_layer,  but  more  usually  from  the  corium,  and  when 
found  in  the  epidermis,  as  it  commonly  is,  it  is  more  likely  to 


THE    INTEGUMENT   AND   THE    EXOSKELETON       79 

have  wandered  in  from  the  corium  than  to  have  originated 
in  place.  Blood-vessels,  with  the  single  exception  of  the 
pharyngeal  mucous  membrane  of  lungless  salamanders,*  are 
entirely  confined  to  the  corium.  Sensory  nerve  endings  of  the 
simplest  type  are  distributed  freely  through  the  epidermis,  but 
the  more  specialized  forms  remain  in  the  corium,  although 
they  may  be  located  in  papillae  pushed  up  into  the  epidermic 
zone.  The  epidermis  thus  forms  a  bloodless  covering  with  but 
slight  sensitiveness,  the  main  function  of  which  is  to  protect 
the  more  delicate  structures  included  in  the  lower  layer.  Aside 
from  this  general  protection  afforded  by  the  unmodified  epi- 
dermis, both  layers  of  the  skin  have  the  power  of  originating 
hard  parts,  which  enter  into  the  formation  of  certain  acces- 
sory external  structures  that  form  a  more  or  less  complete 
exoskeleton.  Thus  the  corium  produces  true  "bone,  with  the 
haversian  canals  and  other  osseous  characters,  while  the  epi- 
dermis forms  hamjmd  enamel,  the  latter  superficially  resem- 
bling bone,  but  harder  and  with  a  different  structure.  The 
structures  formed  from  these  may  be  composed  of  one  sub- 
stance and  involve  but  a  single  layer  in  their  formation,  al- 
though the  other  usually  cooperates  in  some  other  way,  or 
again  may  be  composites  formed  from  material  furnished  by 
both  layers 

Thus  exclusively  horny  structures,  such  as  hairs  or  feathers, 
are  formed  frorjuthe  epidermis  alone,  but,  through  the  neces- 
sity of  nourishment,  they  dip  down  into  the  richly  vascular 
corium,  which  forms  special  organs  to  further  this  result.  The 
dermal  scutes  of  ganoids,  and  the  dermal  bones  of  higher 
forms  arise  wholly  within  the  corium,  while  a  tooth  is  a  com- 
posite structure  composed  of  dentine,  a  hard  sort  of  bone, 
from  the  corium,  overlaid  with  enamel  from  the  epidermis. 

As  exoskeletal  structures  are  universal  among  vertebrates, 
and  often  form  their  most  obvious  characteristics,  and  espe- 
cially as  they  have  interesting  morphological  histories  of  their" 
own,  they  deserve  special  treatment,  and  will  be  taken  up  in 
the  order  of  their  appearance. 

*  See  Chapter  VII,  under  Respiration. 


8o 


HISTORY   OF   THE    HUMAN    BODY 


The  indifferent  or  generalized  condition  that  serves  as  the 
starting  point  for  all  exoskeletal  elements  is  found  in  the  body 
•covering  of  the  dog-fish,  which  consists  of  imbricated  rows  of 
pointed  scales,  that  is,  rows  arranged  in  such  a  way  tHat  the 
scales  of  one  row  cover  the  intervals  of  the  one  behind  it.  This 
typical  arrangement  is  seen  also  in  the  scales  of  other  fishes 
and  reptiles  and  in  the  feather  papillae  on  the  skin  of  a  plucked 


FIG.  18.  Comparison  in  development  and  structure  between  a  placoid 
scale  and  a  tooth. 

(a),  (b),  and  (c)  represent  the  scale;  (d),  (e),  and  (f)  the  tooth.  In  all  the 
figures  the  stratum  corneum  is  dotted,  the  stratum  germinativum  is  represented  by  a 
layer  of  large  cells  with  nuclei;  and  the  cutis  is  presented  as  composed  of  fibers 
with  scattered  cells. 

x,  enamel  membrane;  y,  cutis  papilla;  e,  enamel;  d,  dentine;  p,  pulp  cavity. 

bird.     A  similar,  though  less  obvious,  plan  underlies  the  ar- 
rangement of  the  hair  in  mammals,  as  will  be  shown  later. 

The  scales  in  the  dog-fish  are  of  the  form  known  as  placoid, 
each  consisting  of  an  approximately  flat  base  from  which  rises 
a  sharp-pointed  cusp,  inclined  in  the  direction  of  the  free  edge 
of  the  scale,  or  posteriorly  when  the  scale  is  in  place.  This 
scale  is  somewhat  complex  in  structure  and  consists  of  a  basis 
or  core  of  dentine  overlaid  by  a  layer  of  enamel,  especially 
thick  over  the  cusp,  which  is  almost  wholly  composed  of  it. 
The  scale  is  hollow  beneath  and  a  nutrient  papilla  formed 
from  the  corium  finds  its  way  into  the  interior.  It  arises  in 


THE    INTEGUMENT   AND    THE    EXOSKELETON       81 

the  skin  of  the  embryo  from  a  fold  which  involves  about 
equally  both  layers,  and  the  scale  develops  between  them,  the 
dentine  being  formed  from  the  corium  and  the  enamel  from 
the  epidermis. 

In  selachians  the  jaws  are  equipped  with  several  rows  of 
pointed  teeth,  usually  arranged  like  the  scales  which  cover  the 
surface,  and  as  the  former  have  exactly  the  same  embryonic 
history  as  the  latter  and  are  composed  of  the  same  two  layers, 
it  must  be  concluded  that  they  were  once  simple  placoid  scales 
like  the  rest,  and  that  their  later  modifications  have  been  due 
to  the  difference  of  the  function  to  which  they  have  become 
subjected,  an  inference  sufficient  to  account  for  their  slight 
changes  in  form  as  well  as  for  their  increased  size  and  hard- 
ness, which  is  correlated  with  the  greater  amount  of  work  to 
be  accomplished.  These  teeth,  seen  here  almost  at  their  point 
of  departure  from  generalised  placoid  scales,  are  inherited  by 
all  higher  vertebrates,  although  in  some  cases,  like  turtles  and 
birds,  they  have  become  secondarily  lost.  Aside  from  the 
correspondence  in  form,  arrangement  and  structure,  the  ho- 
mology  is  clearly  shown  by  the  development,  which  proceeds  in 
all  cases  from  a  fold,  involving  both  corium  and  epidermis, 
in  which  the  tooth  subsequently  appears.  These  organs,  when 
once  acquired,  are  subjected  to  great  variations  as  an  accom- 
modation for  the  prehension  and  mastication  of  the  innum- 
erable kinds  of  food;  they  develop  as  pointed  needles,  fangs 
for  inoculating  poison,  sharp-edged  chisels,  flat  surfaces  for 
grinding,  and  ornamental  tusks,  in  all' retaining  the  general 
structure  characteristic  of  placoid  scales.  The  morphology 
of  the  teeth  will  be  taken  up  somewhat  more  at  length  in  the 
chapter  on  the  digestive  system,  with  which  those  parts  be- 
come so  early  associated. 

In  ganoids,  to  which,  as  the  lineal  descendants  of  sela- 
chians, one  should  look  for  the  next  phase  of  this  history,  the 
scales  develop  from  the  corium  alone,  the  epidermis  remain- 
ing passive.  There  is  thus  formed  a  type  of  scale  that  is  com- 
posed entirely  of  dentine,  and  lacks  all  trace  of  enamel.  This 
dentine,  however,  is  very  fine  and  hard  in  character  and  usu- 


82 


HISTORY   OF   THE    HUMAN    BODY 


ally  presents  an  extremely  smooth  and  polished  surface  which 
has  been  often  referred  to  as  genuine  enamel.     Scales  of  this 


B 


D 


FIG.  19.  Dorsal  views  of  various  skulls,  showing  the  dermal  bones. 

(A)    sturgeon     (Acipenser).      (B)     salamander     (Amllytfoma).      (C)     turtle.     (D) 
sea-lion   (Otaria). 

ROS,  rostral  plates;  N,  nasal;  F,  frontal;  Pr.  F,  pre- frontal  Post.  Fr,  post-fron- 
tal; PMX,  pre-maxillary;  MX,  maxillary;  J,  jugal;  QJ ',  quadrato-jugal;  P,  parieialj_ 
SQ,  squamosal;  PT,  pterygoid;  PO,  pro-otic;  OO,  opisth-otic;  SO,  supra-occipital; 
Oc.  Lot.,  lateral  occipital;  OP,  opercular;  S.  Cl.,  supra-clavicle. 

type,  termed  ganoid  (i.  e.,  shining;  from  which  the  Order  re- 
ceives its  name),  are  usually  rhomboid  in  shape  and  lack  the 


THE    INTEGUMENT   AND   THE   EXOSKELETON       83 

cusp  or  point  of  the  placoid  type.  In  the  sturgeon  the  scales 
consolidate  into  large,  bony  shields  or  scutes,  and  this  principle 
was  carried  to  the  extreme  in  the  long  extinct  and  nearly  re- 
lated groups  of  placoderms,  where  the  entire  fish  was  covered 
with  a  heavy  suit  of  mail,  probably  as  a  protection  from  the 
huge  molluscan  forms  which  then  thronged  the  seas.  In  the 
sturgeon,  however,  these  plates  are  not  continuous,  but  are 
arranged  in  longitudinal  rows  along  the  back  and  sides,  leav- 
ing large  areas  unprotected.  In  all  ganoids  similar  scutes 
cover  the  entire  head,  and  fit  together  by  their  edges,  forming 
sutures,  but  leaving  no  appreciable  intervals.  These  are  fairly 
definite  in  number  and  arrangement  in  the  different  species 
and  form  the  so-called  dermal  bones  of  the  skull.  [Cf.  Chap. 
V.]  Certain  of  these,  as  the  / 'rentals,  parietals,  ma.nllaries 
and  squamosals,  persist  in  the  highest  groups ;  others,  like  the 
opercular  and  rostral  series,  disappear  completely,  while  of  an 
extensive  orbital  series  one  alone  persists  as  the  lacrimal. 
The  dermal  bones  that  line  the  mouth  cavity,  such  as  the 
vomers,  palatines  and  parabasal,  retain  the  indications  of  their 
origin  longer  than  do  the  others,  since,  in  many  cases  among 
both  fishes  and  amphibians,  they  are  covered  with  teeth  which 
are  arranged  in  imbricated  series  over  a  considerable  area; 
occasionally,  even,  as  in  the  vomers  of  the  frog  larva,  these 
elements  begin  as  separate  conical  teeth  which  fuse  secondarily 
to  form  the  plate,  thus  repeating  ontogenetically  their  mode  of 
origin.  Nearly  always,  however,  the  history  is  curtailed,  and 
the  dermal  bones  first  appear  as  thin,  lace-like  structures,  lying 
in  the  sub-cutaneous  connective  tissue,  and  enlarge  from  defi- 
nitely located  "  centers  of  ossification  "  by  marginal  additions. 

The  scales  of  teleosts  are  developmentally  and  structurally 
like  those  of  ganoids,  the  ancestors  of  the  group,  but  in  form, 
although  often  rhomboid  in  the  young,  they  become  approxi- 
mately circular,  and  are  hence  termed  cycloid.  Ctenoid  scales 
are  a  variety  of  this  in  which  the  inner  margin  attached  to  the 
skin  is  extended  into  numerous  small  processes  like  the  teeth 
of  a  comb. 

Modern  amphibians  have  a  soft,  slimy  skin,  without  exo- 
skeletal  structures  of  any  kind,  save  in  the  rare  order  of  ccecil- 


84  HISTORY   OF    THE    HUMAN    BODY 

ians  (Gymnophiona),  in  which  scale  rudiments  lie  in  pits  sunk 
beneath  the  surface.  In  the  extinct  group  of  Stegocephali, 
which  possessed  many  amphibian  characters,  the  body  was 
covered  ventrally  with  well-developed,  imbricated  scales.  These 
facts  together  furnish  sufficient  proof  that  amphibians  were 
originally  scaly  and  that  the  present  naked  condition  is  due  to 
a  secondary  reduction.  These  scales  were  probably  bony,  like 
those  of  ganoids  and  teleosts. 

It  is  an  abrupt  transition  from  the  scales  of  fishes  to  those  of 
reptiles,  since,  in  this  latter  class,  the  scales  are  purely  epi- 
dermic in  origin  and  are  composed  of  horn  (keratin),  a  sub- 
stance allied  to  enamel,  without  trace  of  bone.  The  corium, 
it  is  true,  nourishes  the  scales  by  means  of  richly  vascular 
papillae  placed  beneath  each,  but  furnishes  none  of  the  hard 
parts.  There  is  no  doubt  that  in  some  way  these  scales  must 
be  related  to  the  bony  ones  of  ganoids  and  teleosts,  but  the 
relation  appears  to  be  an  indirect  one.  They  may  have  had 
a  common  origin  in  scales  which,  like  those  of  the  placoid 
type,  possess  both  elements,  the  one  emphasizing  the  epidermic 
portion,  the  other  that  of  the  corium.  This  would  seem  to  con- 
flict with  the  direct  derivation  of  reptiles  from  the  ganoids 
as  we  know  them,  and  shows  the  incompleteness  of  our  records 
at  this  point. 

Aside  from  scales  the  reptilian  integument  possesses  a  great 
variety  of  other  exoskeletal  forms,  such  as  spines,  combs,  and 
claws,  all  made  of  keratin,  and  equally  unlike  anything  in 
ganoids  or  amphibians.  In  this  wealth  of  horny  exoskeletal 
elements  the  reptiles  are  closely  followed  by  their  lineal  de- 
scendants, the  birds,  where  the  scales  are  represented  by  the 
far  more  elaborate,  but  strictly  homologous,  feathers,  and 
where  beak  and  feet  are  encased  in  horny  coverings.  The  cov- 
ering for  the  beak  has  evidently  replaced  teeth,  as  in  turtles, 
and  is  undoubtedly  a  recently  acquired  character,  since  fossil 
birds  occur  in  the  Cretaceous  formation,  in  all  respects  like 
modern  birds  save  in  the  presence  of  conical  teeth  set  in 
sockets ;  furthermore,  tooth  germs  have  been  found  in  the  jaws 
of  the  embryos  of  several  species  of  modern  birds,  transitory 


THE    INTEGUMENT   AND    THE    EXOSKELETON       85 

in  character  and  never  developing  far  enough  to  break  through 
the  gums. 

The  scales  of  mammals  are  commonly  little  emphasized, 
owing  to  the  conspicuous  nature  of  the  hairy  coat,  principally 
associated  with  them,  but  they  are,  nevertheless,  of  great  mor- 
phological value.  They  occur  in  definite  regions  and  only  in 
certain  forms,  but  are  so  widely  distributed  that,  were  all 
other  reasons  absent,  their  former  more  extensive  distribu- 
tion would  be  strongly  suggested.  In  most  cases  they  are 
found  only  on  tails  and  paws,  but  in  the  Manidce,  an  edentate 
group,  the  entire  dorsal  surface  of  the  body  and  limbs  is  cov- 
ered with  large,  imbricate  scales,  and  in  the  closely  related 
armadillos,  similar  scales  fuse  to  form  a  dorsal  carapace,  as 
well  as  shields  for  the  head,  tail  and  limbs.  Scale  formation 
on  paws  and  tail  occurs  mainly  in  marsupials,  rodents  and 
insectivores,  and  may  be  seen  particularly  well  on  the  dorsal 
surface  of  the  paws  of  moles  and  shrews,  or  on  the  flat  tails 
of  the  beaver  and  muskrat,  in  which  the  scales  are  usually 
rounded  and  regularly  imbricated.  Where  the  tail  is  cylin- 
drical, as  in  the  rats  and  mice,  the  scales  are  arranged  in  rings, 
those  of  one  row  standing  in  imbricated  relation  to  the  one 
which  it  overlaps. 

In  structure  the  scales  are  epidermic,  like  those  of  reptiles, 
underlaid  by  corium  papillae.  They  usually  remain  more  or  less 
embryonic,  and  the  epidermis,  though  cornified,  does  not  de- 
velop definite  hard  parts,  but  in  the  Manidce  distinct  horn 
scales  are  produced,  as  thick  and  heavy  as  those  of  reptiles, 
the  main  difference  being  that  there  are  here  no  periodic 
ecdyses,  and  the  scales  are  shed  and  renewed  singly,  as  occa- 
sion requires.  In  young  armadillos  the  scales  that  form  the 
carapace  and  shields  are  like  those  of  Manis,  but  they  become 
soon  reinforced  by  ossifications  of  the  corium,  one  for  each 
scale,  which  enlarge  and  finally  fuse  to  form  an  osseous  sub- 
structure. These  corium  elements  are  plainly  secondary  struc- 
tures and  are  not  to  be  considered  as  primary  elements  of  the 
mammalian  scale,  which,  as  stated  above,  is  entirely  epidermic. 

Aside    from   the    sporadic   occurrence   of   scaled    areas    in 


86 


HISTORY   OF   THE    HUMAN    BODY 


various  mammals,  as  previously  noticed,  a  more  definite  proof 
of  the  former  completeness  of  the  scaly  coat  is  found  in  the 
relationship  between  scales  and  hairs  in  scaled  areas,  and  in 


a 


FIG.  20.  Hair  arrangement  in  various  mammals.  Diagrammatic.  [After 
DE  MEIJERE.] 

(a)  Myopotamus  (South  American  rodent).  Tail,  with  scales  and  hairs,  (b)  Mid:is 
(Brazilian  monkey).  Back.  (c)  Sus  vittatus  (pig).  Back.  The  finer  bristles  are 
left  out  upon  the  right  side  of  the  picture,  (d)  Ccelogenys  paca  (the  "  paca,"  a 
South  American  rodent).  Back,  (e)  Dasyurus  viverrinus  (Australian  marsupial). 
Back,  (f)  Loncheres  cristata  (South  American  rodent,  allied  to  Myopotamus).  Back. 

the  arrangement  of  the  hair  in  other  places.     If  almost  any 
scaled  surface  be  examined,  the  tail  of  the  rat  for  example,  it 


THE    INTEGUMENT   AND    THE    EXOSKELETON       87 

will  be  noticed  that  scattered  hairs  appear  among  the  scales 
in  a  definite  relationship,  and  that  a  group  of  three  hairs,  one 
median  and  two  lateral,  projects  from  beneath  the  margin  of 
each  scale,  the  median  hair  being  somewhat  longer  and  stouter 
than  the  others.  It  further  appears  that  there  is  a  similar  ar- 
rangement of  hairs,  usually  in  groups  of  three,  upon  hair  areas 
not  associated  with  scales,  the  hair  groups  being  arranged  in 
imbricated  series,  and  that  this  arrangement  is  general,  even  in 
mammals  without  trace  of  scales.  There  are  some  modifica- 


*e« 


&  © 


FIG.  21.  Hair  arrangement 

£\     ,'."..     •'«;•;  '&      ,-v       in  various  mammals. 

\i?    C  • ;     -- y      <v  v*3> 

(a)       Ursus      arctos      (brown 

/TN      ^-..      ,.£•-.        ~£j,  '';"•        *{•$$  bear).      Front     of     chest.      Dia- 

V^-5     C.*-;      '-^        V*x'  grammatic.       [After      DE      MEI- 

^  ,-;v               .,...      JERE.]       (b)       Cants       familiaris 

(T)          V.~-.       v*':   ..^  -     v***'?     (d°g)«         Four       developmental 

^'  ,;.t      '  •}  stages.      The       adult       arrange- 

'£/  .^-.                     ment      is       like      that      of      (a). 

S     (^       .-v.            .'7  Vl'                     [After         DE        MEIJERE.]     (c) 

i.V>  Homo.     Scalp    of     negro.     Cam- 

— ,.  ^^                   era     drawing     from     the     actual 

•"•    -V>      -^        «>*J1V  ;%.'                  object. 

fl   v./     f  O 


tions  of  this,  due  to  secondary  changes,  such  as  the  need  of  a 
thick  fur,  but  even  in  these  modifications  the  original  plan 
is  still  apparent.  Thus,  in  the  pig,  there  are  two  sets  of  bris- 
tles, a  coarser  set  arranged  in  imbricated  groups  of  three,  and 
a  finer  set,  filling  the  intervening  areas  without  definite  ar- 
rangement; to  obtain  a  thick  fur,  as  in  the  rabbit,  each  hair 
in  the  group  may  become  a  bundle  of  hairs,  the  bundles  being 
arranged  in  groups  of  three  as  in  the  typical  case;  even  the 
number  three  is  not  always  kept,  and  groups  of  five  occur, 


88 


HISTORY    OF    THE    HUMAN    BODY 


with  two  lateral  hairs  on  each  side,  or  groups  of  seven  with 
three.  Occasionally,  as  in  the  dog  and  cat,  the  plan  becomes 
partly  obscured  in  the  adult,  but  is  evident  during  development, 


FIG.  22.  Formation  of  friction  ridges  from  single  rows  of  epidermic 
warts.  [After  Miss  WHIPPLE.] 

(a)  Midas  rosalia  (Brazilian  monkey).  Proximal  portion  of  hypothenar  pad.  (b) 
Midas  rosalia.  Apical  pad.  (c)  Homo.  Advanced  fetus.  Side  of  finger  in  tran- 
sition region.  The  dotted  lines  indicate  the  position  of  sweat-glands. 

the  three-hair  group  being  definitely  marked  in  the  advanced 
fetus. 

A  still  further  corroboration  of  the  former  presence  of  scales 
in  mammals  may  be  obtained  from  the  study  of  the  lower 
surfaces  of  the.paws,  where,  except  in  such  extreme  modifica- 
tions as  the  ungulates,  scales  either  still  exist  or  have  left  a 
permanent  record  in  a  peculiar  configuration  of  the  epidermis, 


THE    INTEGUMENT    AND    THE    EXOSKELETON       89 


FIG.  23.  Formation  of  friction  ridges  in  pairs  from  rings  formed  by 
the  confluence  of  epidermic  warts.  [After  Miss  WHIPPLE.] 

All  the   figures   are   taken   from   Lemur   macaco    (semi-ape    from    Madagascar). 

(a)  Detail  of  area  below  the  interspace  between  index  and  medius.  Here  are 
seen  individual  isolated  warts  w;  groups  of  these  forming  rings  g;  fully  formed 
rings  r;  also  the  formation  of  ridges  in  pairs  by  the  lengthening  of  rings  in  one 
direction  (best  shown  at  the  right) ;  the  single  isolated  ring  enclosed  by  the  ridges 
of  the  pattern  is  also  suggestive,  (b)  A  portion  of  the  interdigital  pad.  (c)  Apical 
pad.  (d)  Detail  showing  two  methods  of  formation  of  three  ridges  from  the  rings. 


90  HISTORY    OF    THE    HUMAN    BODY 

directly  traceable  to  them.  In  their  simplest  form  the  scales 
or  scale  rudiments  are  in  the  form  of  rounded,  wart-like  epi- 
dermic -organs,  which  cover  the  entire  surface  in  Ornitho- 
rhynchus  and  large  portions  of  it  in  marsupials,  insectivores 
and  lemurs.  They  possess  a  more  or  less  imbricated  arrange- 
ment, and  their  identity  with  scales  is  shown,  not  merely  by 
their  structure  and  development,  but  by  a  comparison  with  the 
scaled  dorsal  surface  of  the  paw  in  such  cases  as  that  of  the 
shrew  or  the  star-nosed  mole,  where  the  transitions  from  one 
form  to  the  other  may  be  seen  along  the  edges  of  the  paw. 
This  primitive  condition  is  modified  in  most  cases  by  the  pres- 
ence of  characteristic  mammalian  organs,  the  pads,  which 
are  used  as  contact  surfaces,  and  are  typically  eleven  in  num- 
ber, five  for  the  tips  of  the  digits,  four  for  the  distal  margin 
of  palm  or  sole,  below  the  interdigital  intervals,  and  two  near 
the  wrist  or  ankle.  Upon  these  the  scale  rudiments  become 
arranged  in  rows,  and  by  their  fusion  form  friction  ridges, 
so  called  from  their  use,  which  is  to  prevent  slipping,  like  the 
parallel  ridges  seen  on  the  handles  of  certain  steel  instruments. 
These  friction  ridges  are  always  arranged  at  right  angles  to 
the  direction  in  which  there  is  the  greatest  tendency  to  slip, 
that  is,  directly  across  the  pads  in  walking  forms,  but  arranged 
in  concentric  circles  about  the  highest  part  of  the  pad  in  the 
arboreal  lemurs  and  monkeys  where  slipping  in  all  directions 
is  equally  to  be  expected.  Owing  to  the  general  principles  that 
the  separate  scale  rudiments  form  friction  ridges  on  the  ac- 
tual contact  surfaces  only,  it  follows  that  when  the  pads  re- 
main high  their  surfaces  alone  are  ridged,  while  the  depressed 
areas  are  covered  with  separate  units,  but  when,  as  in  lemurs 
and  monkeys,  there  is  a  progressive  tendency  to  utilise  the 
entire  surface  for  contact,  the  ridged  areas  spread  in  exact 
correspondence  with  the  acquirement  of  contact  surface,  un- 
til, in  the  higher  primates,  the  entire  ventral  surface  of  the 
paws  becomes  covered  with  ridges,  leaving  separate  scale  rudi- 
ments only  along  the  boundaries,  where  this  modified  skin 
meets  that  of  the  dorsal  surface. 

Had  the  friction  ridges,  which  completely  cover  the  palmar 


THE    INTEGUMENT    AND    THE    EXOSKELETON       91 

and  plantar  surfaces  in  Man  and  the  other  higher  primates 
developed  primarily  in  forms  in  which  the  entire  surface  was 
used  for  contact,  it  may  be  assumed  that  they  would  have  taken 
some  simple  form,  designed  with  reference  to  the  area  as  a 
whole;  since,  however,  they  have  passed  through  the  longer 
and  more  complex  history  caused  by  the  introduction  and 
secondary  reduction  of  various  pads,  they  have  preserved  the 
indications  of  the  former  relief  by  an  arrangement  otherwise 
without  cause  or  meaning.  This  may  be  seen  by  a  compari- 
son of  the  lower  surface  of  the  paw  in  some  animal  in  which 
the  pad  system  is  in  full  function  with  that  of  one  in  which 
the  inequalities  of  the  surface  have  become  secondarily  re- 
duced. (Fig.  24.)  The  one  is  an  actual  relief,  the  other  a 
flat  sketch;  the  one  possesses  raised  pads  surrounded  by  folds 
of  skin  which  diverge  in  three  directions  from  points  known  as 
triradii,  the  other  indicates  the  former  location  of  the  pads  by 
whorls  and  other  patterns,  and  that  of  the  folds  by  the  arms 
of  embracing  triradii.  Thus  in  the  field-mouse  (Fig.  24,  a) 
there  are  present  four  interdigital  pads,  the  first  situated  im- 
mediately below  the  interval  between  thumb  and  index,  the 
second  below  the  interval  between  the  latter  and  digit  III, 
and  so  on.  Each  of  these  is  inclosed  by  folds  of  skin  which 
diverge  in  three  directions  from  points  known  as  triradii,  and 
there  are  three  triradii  about  each  pad  except  the  third,  which 
possesses  a  fourth  one,  located  between  digits  III  and  IV. 
Below  these  lie  the  thenar  and  hypothenar  pads,  the  folds  of 
which  are  often  well  marked,  though  not  especially  so  in  this 
case.  The  apical  pads  at  the  ends  of  the  digits  also  possess 
folds,  not  well  shown  in  the  figure,  with  two  triradii,  one  upon 
each  side.  Turning  now  to  the  paws  of  Macacus,  a  small 
monkey  (Fig.  24,  b),  in  which  the  relief  has  been  reduced  to 
a  flat  surface,  each  of  the  above  features  (except  the  thenar  in 
this  especial  case)  is  expressed  by  the  configuration  of  the 
ridges,  as  indicated  in  the  figure.  The  ridges  essential  in 
marking  the  palm  are  represented  by"  solid  lines,  although  in 
reality  not  different  from  the  rest.  Each  pad  is  represented 
by  a  figure  or  pattern,  of  which  the  four  interdigital  are  the 


HISTORY    OF   THE    HUMAN    BODY 


most  typical,  and  are  in  the  form  of  concentric  circles,  the 
center  coincident  with  the  summit  of  the  pad.    The  spiral  form 


II 


IV 


FIG.  24.  Ventral  surface  of  anterior  chiridium  of  an  insectivore  and  of 
a  primate  showing  correspondence  between  relief  and  arrangement  of 
friction  ridges.  [After  Miss  WHIPPLE.] 

(a)  Crocidura  caerulea  (shrew-mouse).  Fore  paw  showing  walking-pads  enclosed 
by  triangular  folds  of  skin,  (b)  Macacus  sp?  (Old  World  monkey).  Hand,  covered 
by  friction  ridges,  the  arrangement  of  which  corresponds  to  the  relief  of  (a). 
The  pads  are  represented  by  concentric  circles,  and  the  triangular  folds  by  triradii. 
These  latter  features  are  here  designated  by  heavy  lines,  although  in  the  real  object 
they  are  not  more  conspicuous  than  the  others. 

of  the  hypothenar  is  a  degeneration  from  the  primitive  type, 
to  which  it  is  connected  by  the  existence  of  transitional  forms, 
either  in  other  individuals  of  the  same  species  or  in  different 


THE    INTEGUMENT   AND    THE    EXOSKELETON       93 

species.  The  thenar  pattern  has  here  become  entirely  re- 
duced, but  is  often  present.  The  apical  patterns  are  also 
modified,  but  in  a  lateral  view  would  show  the  triradii. 

Inasmuch,  however,  as  in  the  Primate  hand  and  foot  the 
ridges  are  still  of  considerable  functional  importance,  they 
are  apt  to  become  modified  at  each  point  of  the  surface  in  ac- 
cordance with  the  use  of  that  point,  and  it  thus  happens  that 
in  different  species  and  in  different  parts  of  the  surface  there 
are  varying  degrees  of  faithfulness  to  the  ancient  records. 
Thus  in  Macacus  the  use  of  the  hand  is  such  that  thenar, 
hypotheriar  and  apical  pads  tend  to  degenerate,  while  the  in- 
terdigitals  are  preserved  in  their  typical  relations,  while  in  the 
human  hand  the  reverse  is  the  case,  and  the  apical  patterns  are 
nearly  always  well  marked,  and  often  in  the  form  of  typical 
whorls  with  two  lateral  triradii ;  while  the  interdigital  patterns 
are  usually  lost  or  obscurely  indicated.  A  hypothenar  pattern 
is  frequent,  especially  in  the  white  race,  and  occasionally  oc- 
curs as  a  whorl  with  three  triradii ;  but  the  thenar  is  of  rare 
occurrence,  and  then  usually  associated  with  the  first  interdigi- 
tal. These  changes  are  in  part  explained  by  the  tendency  of 
the  ridges  to  assume  an  approximately  transverse  direction, 
a  tendency  in  which  the  right  hand  has  surpassed  the  left, 
owing  to  the  long  preferential  use  of  the  former. 

In  the  human  foot  the  apical  patterns  are  about  as  well 
marked  as  in  the  hand,  but  with  a  smaller  percentage  of  the 
primitive  whorl  type ;  the  four  interdigital  pads  are  fairly  well 
indicated  and  often  appear  in  infants  as  rounded  elevations. 
Of  .these  the  most  constant  is  .the  first,  placed  on  the  balPof 
the  foot  below  the  great  toe,  and  is  frequently  of  the  whorl 
type,  occasionally  with  three  triradii;  the  primitive  condition 
again  corresponding  to  the  functional  importance  of  the  region 
which  here  bears  the  main  force  of  the  body  during  a  portion 
of  each  step.  The  hypothenar  is  occasionally  indicated  by 
a  loop  on  the  outer  edge,  but  the  thenar  is  practically  lost. 
An  additional  loop,  of  uncertain  morphological  significance, 
occasionally  occurs  on  the  heel. 


94 


HISTORY   OF   THE    HUMAN    BODY 


FIG.  25.  Print  of  right  hand  of  boy  (Anglo-American),  showing  a  com- 
plete set  of  friction-skin  patterns. 

I,   II,  III,   IV,   the   four   interdigitals;   a,   the  thenar;   b,  the   hypothenar;   the  five 
apical   patterns    (not  lettered)    are   seen  on  the  finger-tips. 


THE    INTEGUMENT   AND    THE    EXOSKELETON       95 

It  thus  appears  that,  aside  from  the  sporadic  distribution  of 
scales  in  various  mammals,  the  palmar  and  plantar  surfaces, 
save  in  the  most  modified  cases  (Ungulates,  Cetacea),  gre^ 
covered  with  scale  elements,  either  distinct  or  united  in  rows 
to  form  ridges;  and,  furthermore,  that  in  other  parts  of  the 
body  the  hair  follicles  occur  in  definite  groups,  arranged  in 
alternate  series;  facts  that  can  be  interpreted  only  as  indica- 
tive of  the  former  presence  of  a  scaly  coat. 

That  this  stage  is  actually  passed  through  in  embryo  mam- 
mals has  not  as  yet  been  definitely  determined,  but  some  cir- 
cumstances seem  to  indicate  that  the  vestiges  of  this  covering 
may  be  looked  for  in  the  epitrichium,  which  is  a  superficial 
epidermic  formation  without  definite  structure  so  far  as  is 
known.  This  at  one  time  covers  the  surface,  but  save  in  the 
palmar  and  plantar  regions  disintegrates  and  disappears ;  con- 
tributing in  man  to  the  formation  of  the  vernix  caseosa,  found 
upon  the  surface  of  the  new-born  infant.  Upon  the  palms 
and  soles,  however,  at  least  in  man,  where  it  has  been  mainly 
studied,  it  appears  to  persist  and  take  part  in  the  formation 
of  the  friction  ridges. 

This  brings  with  it  the  suggestion  that  the  epitrichium  rep- 
resents the  primitive  scaly  coat  of  ancestral  mammals, 
greater  part  of  which  becomes  lost  by  an  embryonal  ecd\sis. 
How  this  epitrichium  appears  and  what  its  fate  is  on  surfaces 
where  scales  persist,  other  than  the  palms  and  soles,  or  in  the 
few  scaled  mammals,  is  not  known;  but  in  the  sloths  and 
ant-eaters,  nearly  related  to  the  last,  it  is  especially  firm  and 
remains  until  birth  as  a  definite  covering.  In  many  mammals 
the  similarity  to  a  moulting  external  layer  is  increased  by  the 
presence  of  a  thick  layer  over  the  nails  or  claws,  continuous 
with  the  epitrichium,  and  cast  off  with  it,  the  eponychium. 

The  hair,  which  forms  the  characteristic  coat  of  present-day 
mammals,  may  be  safely  considered  as  once  accessory  to  a 
covering  of  scales,  which  it  has  secondarily  replaced,  as  ex- 
plained in  the  foregoing,  but  this  does  not  account  for  its 
origin,  or  suggest  its  primary  function.  An  attempt  has  been 
made  to  homologize  hairs  with  the  integumental  sense  organs 


96 


HISTORY   OF   THE    HUMAN    BODY 


of  amphibians,  owing  to  a  similarity  in  the  early  stages  of 
development;  but  although  this  view  may  receive  some  little 
extra  support  from  the  considerable  degree  of  sensitiveness 


Str.C 

Str.muc 

Cutisf 
Layerl 


FIG.  26.  Development  of  hair. 

sir.  c,  stratum  corneum;  str.  mite.,  stratum  germinativum;  seb,  sebaceous  gland; 
fol,  follicle;  out,  outer  root  sheath;  in,  inner  root  sheath;  G,  hair  germ;  d,  beginning 
hair;  pap,  corium  papilla. 

which  some  hairs  attain,  the  evidence  in  favor  of  it  is  slight, 
and  the  idea  does  not  .receive  general  credence.  It  seems 
likely  that  the  hair  developed  subsequently  to  the  scales  and 


THE    INTEGUMENT   AND   THE    EXOSKELETON       97 

not  before  them,  and  may  have  exercised  some  function  of 
protection,  possibly  that  of  a  fringe  upon  or  near  the  free 
edges  to  prevent  the  accumulation  of  dirt  in  the  folds  where 
they  overlap,  a  purpose  for  which  organs  of  similar  appear- 
ance but  of  different  origin  are  frequently  employed  in  in- 
sects and  crustaceans. 

In  development  the  hair  is  wholly  epidermic,  formed  by 
the  stratum  germinativum,  but  dips  down  into  the  corium 
in  the  form  of  a  solid  column  of  rapidly  proliferating  cells, 
the  outer  layer  of  which  soon  differentiates  into  a  sheath  or 
follicle,  while  the  inner  cells  become  horny  and  form  a  shaft 
which  projects  beyond  the  surface  and  becomes  the  hair. 
Growth  is  constantly  kept  up  at  the  bottom  of  the  follicle, 
and  proceeds  from  a  small  area  of  actively  proliferating  cells 
which  are  nourished  by  a  corium  papilla  and  form  the  true 
root,  or  matrix,  of  the  hair.  From  an  inspection  of  the  fol- 
lowing figure  (Fig.  26),  it  becomes  evident  that  this  matrix 
is  merely  a  specialized  portion  of  the  stratum  germinativum 
and  that  the  hair  consists  of  the  upper  layers  derived  from  it, 
and  renewed  from  beneath  as  in  the  superficial  skin.  When 
a  hair  is  pulled  out,  the  break  usually  occurs  immediately  above 
the  matrix,  and  the  lost  portion  involves  the  hair,  the  epider- 
mic sheath,  and  quite  often  the  follicular  sheath  as  well,  parts 
that  are  easily  regenerated  so  long  as  the  matrix  remains. 
Associated  with  this  structure  are  typically  two  sorts  of  glands, 
tubular  and  acinous,  which  are  formed  as  outpushings  from 
the  sides  of  the  follicle  and  grow  down  into  the  corium.  These 
develop  in  various  mammals  to  subserve  many  different  pur- 
poses, often  becoming  dissociated  from  the  original  connec- 
tion with  the  hair.  To  these  two  types  all  forms  of  integu- 
mental  glands  occurring  in  mammals  may  be  referred.  Their 
modifications  and  transformations  may  be  considered  later. 

The  occurrence  and  distribution  of  the  hair  are.  in  strict  ac- 
cordance with  the  needs  of  the  animal,  and  show  great  dif- 
ferences, corresponding  to  the  various  environments  to  which 
mammals  have  become  adapted.  The  hair  may  differ  in 
length,  in  caliber,  in  thickness  (/.  e.,  the  number  of  groups  in 


98  HISTORY    OF    THE    HUMAN    BODY 

a  given  area),  in  texture,  or  in  form;  it  may  be  increased  to  a 
thick,  matted  wool,  or  may  show  every  degree  of  reduction 
down  to  a  total  loss.  It  may  develop  into  bristles,  as  in  the 
hog,  or  even  form  spines,  as  in  the  hedgehog  and  porcupine, 
although  this  latter  result  is  usually  brought  about  by  the  con- 
fluence of  numerous  individual  hairs.  The  "  horn  "  of  the 
rhinoceros  is  such  a  structure,  and  not  a  true  horn.  Many 
variations  in  thickness  are  brought  about  by  modifications  in 
the  hair  groups,  or  by  the  interpolation  of  supernumerary  hairs 
independent  of  the  group  system.  In  the  former  case  the  num- 
ber of  single  hairs  in  each  group  may  be  increased,  or  each 
primary  hair  may  be  represented  by  a  bundle ;  or  again,  each 
primary  hair  may  be  accompanied  by  a  series  of  accessory 
hairs,  arranged  as  satellites  about  the  former.  In  the  latter 
case  there  is  usually  a  marked  difference  between  the  hairs 
that  are  included  in  the  primary  system  and  those  that  are  not, 
as  is  seen  in  the  case  of  the  hog,  in  which  there  are  two  sizes 
of  bristles,  coarse  ones  in  groups,  and  finer  ones  interpolated 
without  system  (Fig.  20,  c). 

It  may  be  said  in  general  that  arctic  forms  and  those  liv- 
ing at  high  altitudes  are  the  most  plentifully  supplied  with 
hair,  while  tropical  and  sub-tropical  forms  are  sparsely  cov- 
ered. An  aquatic  life  tends  to  reduce  the  hair  coat;  if  the 
animal  is  but  semi-aquatic,  as  seals  and  otters,  the  hair  is 
reduced  to  the  form  of  a  fine  plush,  but  in  the  Cetacea  and 
Sirenia,  which  are  wholly  aquatic,  the  reduction  is  almost 
a  complete  one.  Many  apes  are  but  scantily  supplied  with 
hair,  the  ventral  side  of  body  and  limbs  being  but  sparsely 
covered,  while  the  upper  part  of  the  face  and  ears  are  nearly 
bare.  The  same  tendency  is  continued  farther  in  Man,  who 
shows  considerable  racial  variation,  ranging  from  the  hairy 
Airius  and  certain  hairy  individuals  in  the  white  race  to  the 
smooth  and  beardless  Malays.  That  Man  was  formerly  sup- 
plied with  a  thick  coat  of  hair,  however,  is  shown  by  the  fetal 
condition,  at  one  stage  of  which  the  entire  body,  not  excepting 
the  face,  is  covered  by  a  coat  of  fine  down,  the  lanugo.  This 
mainly  disappears  before  birth,  and  becomes  eventually  re- 


THE    INTEGUMENT   AND   THE    EXOSKELETON       99 

placed  by  the  permanent  coat,  which  usually  shows  but  slight 
development  save  in  certain  definite  localities.  The  lanugo 
persists  in  a  reduced  condition  on  the  face,  especially  in  females, 
forming  the  down  which  gives  to  the  cheeks  their  character- 
istic bloom.  Abnormal  hairiness  in  man,  or  hypertrichosis, 
is  fortunately  rare,  and  is  of  two  kinds ;  the  one,  hypertricho- 
sis  vera,  is  due  to  an  excessive  growth  of  the  permanent  coat 
which  replaces  the  lanugo;  the  other,  psendohypertrichosis,  is 
the  result  of  the  persistence  of  the  lanugo. 

Localized  hypertrophy  in  various  mammals  in  the  form  of 
manes,  crests  or  tufts  of  hair,  is  of  frequent  occurrence  and 
is  used  for  various  purposes,  such  as  defense  from  flies  or 
other  noxious  insects,  attraction  of  the  other  sex,  or  as  a  pro- 
tection from  the  teeth  of  rivals.  Under  this  general  head 
come  also  the  beard  of  man,  which  corresponds  in  position 
and  direction  to  that  found  in  other  primates,  and  the  long 
hair  of  the  head.  The  other  locations  in  Man  in  which  long 
hair  occurs,  the  axillary  and  pubic  regions,  do  not  seem  to 
belong  here,  and  probably  represent  portions  that  escaped 
reduction  rather  than  hypertrophy.  The  obvious  function  of 
the  cranial  hair  is  a  protection  from  the  sun,  and  its  location 
suggests  that  it  is  developed  with  reference  to  the  erect  and 
not  the  quadrupedal  position,  in  which  latter  case  it  would 
have  extended  farther  down  the  back.  The  axillary  and  pubic 
tufts  may  be  for  lessening  the  friction  between  the  limbs  dur- 
ing motion ;  it  has  been  also  suggested  that  they  possessed  a  use 
in  transitional  forms  in  furnishing  places  to  which  the  infant 
might  cling,  thus  leaving  the  arms  of  the  parent  free  for 
climbing.  In  support  of  this  latter  view  it  may  be  noticed  that 
the  distances  between  these  locations  correspond  approxi- 
mately to  the  proportions  of  a  normal  infant,  and  that  an  in- 
fant thus  attached  is  also  in  the  right  position  for  nursing. 

Aside  from  differences  in  caliber  and  length,  the  hair  of  vari- 
ous mammals  differs  markedly  in  structure,  in  color,  in  the 
shape  of  its  cross-sections  in  various  places  and  in  the  shape 
assumed  by  each  hair.  In  structure  a  hair  consists  of  a  firmer 
cortex  of  varying  thickness  enclosing  a  softer  medulla;  a 


ioo  HISTORY   OF   THE    HUMAN    BODY 

single  layer  of  epidermic  cells  covers  the  cortex  externally. 
Differences  in  color  and  luster  are  due  to  the  amount  of  pig- 
ment in  the  cortex,  the  sculpture  of  the  epidermic  covering  and 
the  presence  or  absence  of  air  in  the  medulla. 

The  cells  of  the  epidermic  covering  may  fit  smoothly  upon 
one  another  or  may  project  like  scales.  A  typical  illustration 
of  this  latter  case  is  that  found  in  wool,  and  by  virtue  of  this 
peculiarity  the  separate  hairs  may  be  made  to  cling  together 
by  causing  the  minute  teeth  to  interlock,  a  result  effected 
throught  the  act  of  spinning.  To  this  peculiarity  the  possi- 
bility of  wool  as  a  textile  fabric  is  due. 

In  Man  there  is  much  racial  variation  in  the  hair  of  the 
head,  a  character  of  considerable  value  in  ethnology.  The 
degree  of  waviness  or  curliness  is  due  to  the  shape  of  the 
single  hairs;  if  they  are  cylindrical,  that  is,  circular  in  cross- 
section,  they  are  perfectly  straight,  as  in  the  typical  Mongo- 
lian ;  a  slight  degree  of  flatness  with  an  elliptical  cross-section, 
allows  the  hairs  to  become  wavy,  as  in  many  Europeans; 
if  more  flat,  they  are  curly,  and  if  very  flat,  the  hairs  are 
woolly,  as  in  the  Negroes.  In  this  last  class  there  are  two 
subdivisions,  the  Eriocomi,  where  the  hair  is  evenly  dis- 
tributed, making  a  solid  mat,  and  the  Lophocomi,  the  "  che- 
veux  en  grains  de  poivre,"  in  which  the  hair  is  collected 
into  little  tufts  with  partings  between  them.  This  latter  pe- 
culiarity is  seen  in  adult  Bushmen  and  Hottentots  and  in  the 
children  of  most  other  negro  races. 

The  degree  of  flatness  of  the  cross-section  is  expressed  by 
an  index  in  which  the  longer  diameter  is  considered  unity 
and  the  shorter  is  compared  with  it  in  the  form  of  a  decimal 
fraction.  Thus,  in  a  perfectly  cylindrical  hair,  the  index 
would  be  ioo,  in  one  in  which  the  breadth  of  the  oval  is  half 
the  length  the  index  would  be  50.  As  a  matter  of  fact  there 
is  no  index  so  high  as  ioo,  but  it  ranges  between  85,  that  of 
the  Japanese,  and  40-50,  that  of  the  Hottentots.  In  Euro- 
peans it  varies  between  62  and  72.  In  length  the  hair  varies 
greatly,  straight  hair  being  the  longest  and  woolly  hair  the 
shortest.  In  races  with  either  extreme  (straight  or  woolly) 


THE    INTEGUMENT   AND   THE   EXOSKELSTON      i6i 

the  hair  of  the  two  sexes  is  of  equal  length,  but  in  those  with 
wavy  or  curly  hair  that  of  the  female  considerably  exceeds  in 
length  that  of  the  male. 

The  hair  exhibits  a  definite  slant  or  direction  of  groivth, 
which  varies  in  different  parts  of  the  body,  so  that  one  may 
speak  of  hair-streams  or  hair-currents.  This  direction  is  the 
one  shown  by  the  follicle  and  by  the  hair  immediately  after 
its  emergence  from  the  skin,  and  is  entirely  unrelated  to  the 
various  directions  which  the  free  masses  may  temporarily 
assume  under  the  influences  of  gravitation,  wind,  or  other 
external  forces.  It  is  thus  best  seen  in  animals  with  a  coat  of 
short,  appressed  hair,  like  horses  or  short-haired  dogs,  and  is 
often  quite  obscured  in  those  with  long  hair,  or  in  those  with 
soft,  plush-like  fur,  like  seals  and  moles.  In  these  latter, 
however,  it  may  be  accurately  ascertained  by  shaving  or  clip- 
ping the  hair. 

In  general  it  may  be  said  that  a  given  area  shows  a  defi- 
nite direction,  the  lines  of  which  may  be  parallel  or  some- 
what divergent,  two  adjacent  areas  being  separated  either 
by  a  parting,  where  the  streams  diverge  from  one  another,  or 
by  a  raised  crest  or  seam  where  they  converge.  At  certain 
points  special  features  may  be  noticed,  the  most  important 
of  which  are  the  vortex  or  whorl,  the  rhomboid  and  the  feath- 
ering. In  the  vortex  variously  directed  hair  currents  unite 
to  form  a  spiral  figure,  which  either  converges  to  form  a 
central  tuft,  convergent  vortex,  or  starts  at  the  center  and 
diverges,  divergent  vortex.  The  first  type  of  vortex  often 
marks  a  point  at  which  some  projecting  organ  is  later  to  ap- 
pear, as  at  the  corners  of  the  forehead  in  the  calf  before 
the  appearance  of  the  horns ;  or  else  one  where  a  former  pro- 
jecting organ  has  disappeared,  as  at  the  umbilicus;  but,  on  the 
other  hand,  there  are  numerous  instances  where  such  a  rela- 
tionship cannot  be  established.  The  significance  of  the  second 
type  is  unknown.  Either  type  may  form  either  a  right-  or  a 
left-handed  spiral  (clockwise  or  contra-clockwise).  A  rhom- 
boid is  an  open  space  of  the  shape  designated  by  the  name, 
and  appears  where  the  corners  of  four  areas  meet.  It  is  thus 


IQ-2 


HISTORY   OF   THE   HUMAN   BODY 


FIG.    27.     Hair   direction   in   human   fetus.     [After  VOIGT.] 

The  black  lines  designate  the  lines  of  parting,  the  arrows  show  the  direction  of 
the  hair  currents.     Rhomboids  and  vortices  are  also  shown. 


THE   INTEGUMENT   AND   THE   EXOSKELETON      103 

always  so  arranged  that  the  hairs  converge  at  two  opposite 
corners  and  diverge  at  the  other  two.  A  feathering  is  a  spe- 
cial form  of  area,  usually  more  extensive  than  the  two  last, 
occurring  only  in  association  with  a  vortex,  of  which  it  forms 
a  continuation  in  one  direction.  It  is  in  the  form  of  a  long 
and  narrow  ellipse,  and  the  hair  currents  run  along  a  central 
axis  and  diverge  to  the  margin. 

All  of  the  above  forms  may  be  readily  seen  upon  our  do- 
mestic animals,  and  are  often  well  marked  in  man,  especially 
in  individuals  whose  skin  is  covered  with  very  short  appressed 
hairs.  An  especially  good  object  is  the  broad,  square  chest 
of  the  bull-dog,  on  which  are  usually  three  vortices  and  three 
rhomboids ;  a  vortex  above  and  a  rhomboid  below  in  the  me- 
dian line,  a  lateral  rhomboid  on  each  side  of  the  vortex,  and 
a  lateral  vortex  on  each  side  of  the  rhomboid.  Aside  from 
these  there  occurs  a  vortex  on  each  elbow,  usually  one  on  each 
side  of  the  neck,  and  upon  the  hinder  parts  a  pair  of  especially 
conspicuous  vortices,  above  which,  at  the  base  of  the  tail, 
are  two  rhomboids.  Individual  variation  may  show  depar- 
tures from  this  description. 

In  Man  the  various  features  are  present  and  often  well 
marked,  but  as  they  require  for  their  expression  a  certain 
grade  of  pilosity,  they  are  usually  overlooked.  Here,  also, 
as  in  other  animals,  there  is  considerable  individual  variation, 
and  a  feature  marked  on  one  person  may  be  absent  on  another ; 
the  two  sides,  also,  are  not  necessarily  symmetrical.  The 
most  conspicuous  vortex  is  the  one  at  the  crown  of  the  head, 
easily  observed  in  boys  with  short  hair.  This  may  be  either 
clockwise  or  contra-clockwise,  and  seems  to  follow  no  rule 
in  this  respect.  Other  vortices  occur  above  the  angle  of  the 
jaw  and  in  front  of  the  axilla.  Rhomboids  occur  along 
the  mid-ventral  line;  one  of  them  is  situated  at  the  angle 
between  the  throat  and  the  chin,  immediately  above  the  thy- 
reoid  protuberance,  a  second  at  the  anterior  end  of  the  sternum, 
and  a  third  on  the  abdomen,  midway  between  the  umbilicus 
and  the  pubic  eminence.  A  rhomboid  is  found  constantly  upon 
the  lower  part  of  the  ulna,  a  little  above  the  wrist. 


104  HISTORY   OF   THE    HUMAN    BODY 

The  study  of  hair  direction  has  excited  an  occasional  inter- 
est among  morphologists,  and  a  number  of  theories  have  been 
advanced  to  explain  the  origin  of  the  various  features,  but 
there  has  been  as  yet  too  little  morphological  work  in  this  field 
to  allow  much  theorizing  or  to  serve  as  a  basis  for  definite  con- 
clusions. The  general  tendency  of  the  hair  to  slope  backwards 
from  the  point  of  the  nose  to  the  end  of  the  tail  suggests  the 
influence  of  the  air-currents  upon  a  rapidly  moving  body,  or 
at  least  an  adaptation  to  them,  the  same  phenomenon  being 
strikingly  exhibited  by  the  direction  of  feathers  in  birds,  and 
that  of  scales  in  reptiles;  in  the  same  way  the  general  down- 
ward slope  of  the  hair  along  the  sides  of  quadrupeds  suggests 
the  influence  of  gravitation,  especially  when  taken  in  connec- 
tion with  the  apparent  hair  direction  in  the  sloth,  which  shows 
a  parting  along  the  mid-ventral  line  and  is  directed  ventro- 
dorsally,  as  if  in  correlation  with  the  customary  inverted  posi- 
tion of  the  animal.  In  opposition  to  this,  however,  it  may  be 
pointed  out  that  in  certain  areas  the  direction  is  the  reverse  of 
that  which  either  of  the  above  forces  would  produce,  and  as 
for  the  case  of  the  sloth,  the  direction  observed  may  be  that 
assumed  by  the  long  hair  after  emerging  from  the  surface, 
since  the  direction  of  the  follicles  seems  never  to  have  been 
investigated.  Darwin's  well-known  attempt  to  attribute  the 
hair  direction  on  the  human  arms  to  the  direct  influence  of 
tropical  rains  upon  the  arms  of  simians,  when  held  above  the 
head  for  protection,  is  at  variance  with  the  facts,  and  hence 
must  be  dismissed  from  the  discussion. 

Recently  a  new  line  of  explanation  has  been  sought  in  the 
influence  of  underlying  parts,  especially  that  of  the  sub-cutane- 
ous muscles,  the  constant  traction  of  which  influences  the  hair 
follicles  over  definite  areas,  but  this  idea  cannot  as  yet  be  con- 
sidered to  have  passed  the  stage  of  a  vague  hypothesis, 
especially  since  many  of  the  observations  are  fallacious,  and 
hence  have  no  weight  in  establishing  the  conclusions. 

It  seems  likely,  since  the  hairs  originated  in  association 
with  a  complete  coat  of  scales  and  at  a  time  which  must  be 
designated  as  premammalian,  and  since  the  original  hair  direc- 


THE   INTEGUMENT   AND   THE   EXOSKELETON      105 

tion  must  have  been  the  same  as  that  of  the  scales  which  pre- 
ceded them,  that  this  original  "direction  would  have  been 
retained  after  the  loss  of  the  scales,  and  that  the  hereditary 
transmission  of  this  may  account  for  at  least  a  general  plan 
underlying  the  variations  occurring  in  the  mammals  of  the 
present  day.  The  existence  of  individual  variations,  known 
to  be  considerable  in  man  and  certain  domestic  animals,  points 
to  a  diminution  of  the  original  functional  importance,  which 
has  become  no  longer  sufficient  to  retain  the  various  features 
at  a  definite  standard. 

Aside  from  the  formation  of  horny  scales,  feathers,  and  hair, 
the  epidermis  produces  numerous  other  organs  composed  of 
keratin,  and  fitted  for  various  uses.  The  most  of  these  appear 
as  isolated  instances  to  subserve  a  particular  purpose  in  a 
small  group  of  animals,  'but  in  one  case,  that  of  claws  or 
nails,  the  organs  are  possessed  by  both  Sauropsida  and  Mam- 
malia and  form  a  strictly  homologous  series  throughout,  which 
presents  some  interesting  modifications. 

The  first  employment  of  this  substance  in  this  locality 
appears  to  be  in  the  dog-fish,  where  the  fins  are  lengthened 
beyond  the  limits  of  the  fish  skeleton  by  numerous  horn 
threads,  set  close  together  and  forming  two  series,  overlapping 
the  cartilaginous  rays  on  each  side.  Otherwise  there  is  little 
use  of  keratin  among  fishes  and  almost  none  at  all  among 
amphibians,  unless  there  be  included  a  certain  form  of  wart 
found  in  toads  and  due  to  the  local  thickening  of  the  stratum 
corneum.  One  species  of  salamander  also  (Siren)  possesses 
horny  plates  in  the  mouth,  serving  the  purpose  of  teeth.  In 
turtles,  a  dorsal  carapace  and  a  ventral  plastron  are  formed 
from  parts  of  the  endoskeleton,  with  the  addition  of  dermal 
elements,  and  these  are  covered  by  large  plates  of  keratin,  the 
so-called  "  tortoise-shell."  The  jaws  of  the  same  animal  are 
also  covered  with  horny  plates  equipped  with  a  sharp  cutting 
edge,  and  a  precisely  similar  formation  produces  the  charac- 
teristic beak  of  birds,  although  it  is  hardly  to  be  supposed  that 
the  two  structures  are  genetically  connected.  Aside  from 
the  coat  of  imbricated  scales,  many  reptiles  possess  horns, 


io6  HISTORY   OF   THE   HUMAN    BODY 

crests  and  other  cornified  structures,  many  of  which  are  un- 
doubtedly scale  modifications;  and  in  birds  there  are  occa- 
sionally horny  structures,  often  with  a  core  of  bone,  like  the 
spurs  of  the  game  cock,  of  doubtful  morphological  value.  The 
lower  legs  and  feet  of  birds  are  encased  by  a  horny  epidermis, 
a  part  of  which  is  covered  by  definite  scales,  while  other  parts 
of  it  are  divided  by  grooves  into  square  or  polygonal  areas. 
The  skin  of  crocodiles  is  marked  in  much  the  same  way  and 
does  not  form  overlapping  scales,  yet  it  is  highly  probable  that 
in  both  cases  the  areas  separated  are  the  equivalent  of  scales, 
since  overlapping  is  not  a  necessary  characteristic  of  these 
organs. 

In  mammals  there  are  many  special  organs  composed  of 
keratin.  The  "  whalebone "  of  whales  is  derived  from  the 
epidermis  of  the  hard  palate  and  forms  a  thick  fringe  which 
hangs  from  the  upper  jaw  and  is  employed  as  a  strainer. 
There  are  three  types  of  horns:  that  of  the  rhinoceros,  formed 
by  a  coalescence  of  numerous  keratin  fibers,  probably  the 
morphological  equivalent  of  hairs;  the  hollow  type  found  in 
some  ruminants,  in  which  a  hollow  keratin  structure  is  fitted 
over  a  core  of  bone;  and,  thirdly,  the  solid  horn  of  deer  and 
antelopes,  where  the  final  structure  is  composed  of  the  bony 
core  alone,  the  epidermis  being  represented  by  the  "  velvet/* 
an  external  covering  which  atrophies  after  the  horn  is  com- 
pleted and  is  rubbed  off  by  the  animal.  Thus  this  last,  in  its 
final  condition,  cannot  be  counted  among  epidermic  structures. 

In  reptiles,  birds  and  mammals  the  ends  of  the  digits  are 
armed  by  horny  structures,  strictly  homologous  throughout, 
although  variously  denominated  as  claws,  nails  or  hoofs,  in 
accordance  with  their  shape.  In  the  typical  claw  (Fig.  28,  a) 
the  parts  to  be  noted  are  the  convex  dorsal  plate  (Krallen- 
platte),  the  concave  ventral  plate  (Sohlenhorn)  and  the  apical 
pads  of  the  digit  (Zehenballen).  In  the  sauropsidan  claw  (a) 
the  two  plates  are  of  about  equal  importance  and  the  terminal 
pad  is  represented  by  an  unmodified  scale  or  by  several 
scales.  In  the  typical  mammalian  claw  (b)  the  ventral  plate 
is  somewhat  reduced  and  the  terminal  pad  is  well  developed 


THE   INTEGUMENT   AND   THE    EXOSKELETON      107 

and  covered  with  friction-ridges.  In  monkeys  (d)  the  dorsal 
plate  is  flatter  and  broader  as  an  adaptation  to  the  prehensile 
hand  or  foot  and  does  not  project  much  beyond  the  end  of  the 
digit;  the  ventral  plate  is  much  reduced  in  extent  and  is  not 
very  horny,  and  the  terminal  pad  has  decreased  in  volume  and 
is  indicated  mainly  by  the  friction-ridges,  which  are  in  the 
form  of  a  loop  or  whorl.  The  extreme  of  this  line  of  develop- 


FIG.  28.  Diagrammatic  longitudinal  sections  through  digits  of  various 
mammals,  to  illustrate  the  morphology  of  claws,  hoofs,  and  nails,  [(a), 
after  GEGENBAUR;  (b)-(e),  after  BOAS.] 

(a)     Echidna,     (b)    Typical   unguiculate.      (c)    Horse,      (d)    Monkey,      (e)   Man. 
The  dorsal   plate  is  represented  by  solid  black;   the  ventral  plate  is  striped.     The 
bones  are  dotted. 

ment  is  reached  by  man  (e)  in  which  the  last  remnant  of  the 
ventral  plate  appears  in  the  narrow  strip  of  skin  between  the 
inner  surface  of  the  nail  (i  e.,  the  dorsal  plate)  and  a  terminal 
fold  where  the  friction-ridges  commence.  The  terminal  pad 
is  much  as  in  monkeys.  In  the  hoofed  quadruped  another  line 
of  development  is  shown  (c)  in  which  the  ventral  plate  forms 
a  horny,  though  rather  soft,  surface  for  contact  with  the 
ground.  There  is  no  pad,  and  the  soft  integument  represent- 
ing that  area  lies  behind  the  hoof,  continuous  with  the  ventral 
plate. 

Glands  occur  in  the  integument  of  all  vertebrates,  profusely 
in  fishes,  amphibians  and  mammals,  rarely  and  strictly  local- 
ized in  the  sauropsida.  They  are  always  derived  from  the 
stratum  germinativum  of  the  epidermis  and  vary  greatly  in 


io8 


HISTORY   OF   THE    HUMAN    BODY 


complexity  of  development  and  in  the  nature  of  their  secre- 
tion. The  principles  underlying  gland  formation  are  very 
simple,  and  may  be  briefly  considered  in  this  place  before 
taking  up  in  detail  their  occurrence  and  distribution  in  verte- 


FIG.  29.  Diagrams  of  various  types  of  glands,  shown  as  invaginations 
from  a  layer  of  indifferent  epithelium. 

(a)  represents  a  region  in  which  certain  of  the  surface  cells  are  differentiated 
as  unicellular  glands.  (b)  is  a  simple  tubular  gland  and  (c)  a  simple  acinous 
gland,  each  formed  from  a  complex  of  gland  cells.  Tubular  glands  may  become  coiled 
(d),  or  branched  (e).  Acinous  glands  may  consist  of  a  single  acinus,  as  in  (c), 
or  of  several,  as  in  (f).  A  still  greater  complexity  is  seen  in  (g),  where  each 
acinus  possesses  its  own  excurrent  duct,  all  being  collected  into  a  common  duct 
which  leads  to  a  single  outlet. 

brate  integument.  The  protoplasm  of  all  cells  has  the  power 
of  storing  up  some  form  of  secondary  material,  metaplasm, 
extracted  from  the  materials  supplied  to  it,  and  a  gland  cell 
differs  from  another  mainly  in  the  fact  that  its  metaplasm  is 
of  use  to  some  other  part  of  the  organism  and  that  its  chief 
value  to  the  organism  lies  in  the  material  which  it  produces. 


THE    INTEGUMENT   AND    THE    EXOSKELETON      109 

Usually  also  a  gland  cell,  specialized  as  it  were  in  this  direc- 
tion, secretes  these  products  in  greater  abundance  than  in  the 
case  of  other  cells.  As  a  single  cell  may  thus  have  all  the  attri- 
butes of  a  gland,  the  simplest  glands  are  composed  of  but  one 
such  elementary  unit  and  are  unicellular.  Such  simple  glands 
are  of  extensive  occurrence  among  animals  and  are  generally 
used  where  a  surface  is  to  be  kept  uniformly  moistened  with 
some  secretion,  as  a  protection  against  water  or  air,  and  \yhere 
there  is  no  special  auxiliary  structure,  like  the  eyelids  of  land 
vertebrates,  to  insure  an  even  distribution.  The  majority  of 
glands,  however,  are  multicellular  and  represent  various  solu- 
tions of  the  problems  of  how  to  increase  the  physiological 
efficiency  within  a  definite  space,  i.  e.,  how  to  increase  the 
effective  secreting  surface  without  increasing  the  mass.  The 
diagrams  in  Fig.  29  represent  various  solutions  of  this  prob- 
lem, as  well  as  varying  degrees  of  physiological  efficiency,  the 
most  complex  form  being  in  general  the  most  successful.  Be- 
ginning with  single  cells  opening  upon  a  free  surface  it  is 
evident  that  the  efficiency  increases  with  the  number  of  gland 
cells  in  a  given  space,  the  limit  of  this  type  being  reached 
when  all  the  cells  have  become  thus  employed.  If,  however, 
the  problem  allows  the  utilization  of  a  certain  amount  of  depth, 
the  efficiency  may  become  much  increased  by  folding  or 
invaginating  portions  of  the  original  surface  below  the  general 
level,  either  in  the  form  of  tubules  (b)  or  flask-shaped  glo- 
bules, acini  (c).  Each  of  these  primary  types  may  become 
still  further  complicated  in  several  ways.  The  tubular  form 
may  become  convoluted  (d)  or  branched  (e),  and  the  acinous 
form  may  develop  secondary  acini  (f).  Through  a  slight 
cellular  differentiation  the  cells  nearest  the  outlet  of  the  gland 
may  become  flattened  and  form  a  non-secreting  duct  through 
which  may  pass  the  fluid  manufactured  in  the  secretory  por- 
tion. This  principle  may  be  extended  to  the  secondary  acini, 
and  when  these  latter  become  profusely  multiplied,  the  result 
is  a  definitely  localized  and  very  effective  organ,  as  in  (g). 
These  varied  forms  are  not  sharply  defined,  and  even  the 
fundamental  types  of  tubular  and  acinous  glands  may  grade 


i  io  HISTORY    OF    THE    HUMAN    BODY 

into  one  another  in  such  a  way  as  to  make  the  classification 
indeterminate. 

Glands  may  be  also  divided  according  to  their  method  of 
furnishing  the  secretion,  since  some  cells,  when  surcharged, 
liberate  their  fluid  by  bursting,  and  thus  become  destroyed, 
while  others  allow  their  secretion  to  pass  through  their  walls, 
retaining  their  physiological  life  for  an  indefinite  period.  In  the 
former  case  the  supply  of  cells  is  kept  up  by  a  constant  prolifer- 
ation from  a  zone  of  growth ;  in  the  other  case  the  community 
of  cells  retains  its  identity.  The  glands  in  the  former  case  are 
termed  necrobiotic,  in  the  other  they  are  vitally  secretory. 
This  physiological  distinction  is  often  of  use  in  determining 
homologies  at  times  when  the  structure  is  non-committal  or 
misleading. 

Unlike  most  other  structures,  the  integumental  glands 
of  vertebrates  do  not  appear  to  have  a  continuous  history 
in  the  various  Classes,  but  are  developed  in  each  Class,  or 
even  in  specific  cases,  to  suit  the  needs  of  particular  environ- 
ment or  habits.  In  fishes  and  amphibians  the  main  function 
of  integumental  glands  is  to  secrete  a  protective  slime,  to 
defend  the  surface  from  the  action  of  the  water,  to  which,  as  a 
secondary  function,  probably  accidental  at  first,  there  is  often 
added  to  the  secretion  some  acrid  or  even  actively  poisonous 
quality,  for  defense  against  predaceous  animals.  The  glands 
supplying  this  function  are  often  of  the  unicellular  type,  with 
a  narrowed  neck  at  the  surface,  and  called  beaker  cells  from 
their  shape;  the  simple  acinous  type,  in  the  form  of  flask- 
shaped  glands,  is  widely  distributed  among  amphibians,  where 
the  glands  often  occur  in  clusters,  causing  a  conspicuous  pro- 
tuberance. The  integument  of  the  Sauropsida  is  characterized 
by  an  almost  complete  absence  of  glands,  certain  special  ones 
appearing  in  definite  localities  and  employed  for  some  special 
purpose.  Such  are  the  cloacal  glands  of  snakes,  which  secrete 
for  defensive  purposes  a  milky  fluid  having  a  nauseating  odor, 
and  the  musk  glands  of  certain  turtles,  which  may  be  defensive 
or  used  as  a  sexual  allurement.  In  the  males  of  certain  lizards 
a  single  line  of  glands  opens  along  a  definite  row  of  scales  on 


THE    INTEGUMENT   AND   THE    EXOSKELETON      in 


the  inner  aspect  of  the  femora;  at  the  time  of  pairing  these 
secrete  a  gummy  fluid  which  hardens  into  short  spines  or  teeth, 
employed  during  copulation.  In  birds  the  sole  integumental 


Stratum  corneum 

Stratum  lucidum      }- Epidermis 
Stratum  mucosum  j 


-  Corium 


FIG.  30.  Typical  mammalian  hair  with  its  accessory  parts.  [After 
WEBER.] 

67.  ac.,  acinous  gland,  usually  sebaceous;  GL  tb.,  tubular  gland,  usually  per- 
spiratory in  function;  M,  ar.  piL,  arrector  pilarum  muscle. 

gland  is  the  uropygial,  a  compound  mass  situated  on  the  dorsal 
aspect  of  the  tail  rudiment,  and  secreting  an  oily  fluid  used 
for  anointing  the  feathers. 

In  mammals,  the  skin  is,  as  a  rule,  profusely  glandular,  and 
the  glands  possess  the  highest  degree  of  physiological  dif- 
ferentiation. In  spite  of  their  diversity,  however,  they  may 


H2  HISTORY   OF   THE    HUMAN    BODY 

all  be  referred  to  two  primary  forms,  each  originally  de- 
veloped in  association  with  a  hair.  This  primitive  condition 
is  still  common,  though  often  with  some  slight  modifications, 
and  is  shown  in  diagrammatic  form  in  Fig.  30.  The  glands 
arise  as  outpushings  from  the  wall  of  the  follicle,  into  which 
they  empty.  One  of  these  types  is  a  long,  slender  tube,  often 
convoluted  at  its  free  end,  a  tubular  gland;  the  other  short 
and  somewhat  lobed,  an  acinous  gland.  Aside  from  this 
morphological  distinction  the  tubular  gland  is  vitally  secre- 
tory, the  acinous  necrobiotic.  As  a  secondary  modification 
either  type  may  exist  without  a  hair,  but  such  cases  are  excep- 
tional. It  seems  likely  that  such  a  complex  as  that  represented 
in  the  figure  was  originally  associated,  as  are  the  hairs,  with 
the  primary  scales,  one  for  each,  the  glands  being  associated 
with  the  median  hair  only,  but  this  cannot  as  yet  be  definitely 
asserted. 

Each  of  the  two  glandular  elements  is  capable  of  great  mod- 
ifications, both  morphologically  and  physiologically.  The 
tubular  type  is  not  always  convoluted,  but  may  be  straight 
and  simple,  or  branched.  Its  characteristic  secretion  is  a  thin, 
colorless,  watery  fluid,  the  perspiration  or  sweat,  but  it  is 
viscous  and  reddish  in  the  hippopotamus  and  albuminous  and 
of  a  blue  color  in  Cephalophus,  a  South  African  antelope.  A 
much  modified  form  of  these  glands  furnishes  the  thick  and 
oily  ear-wax.  In  distribution  these  glands  are  often  found 
over  the  entire  body  (hippopotamus,  bear),  but  may  be  strictly 
localized,  as  in  most  rodents,  where  they  are  found  mostly  on 
the  ventral  surface  of  the  paws.  They  fail  entirely  in  the 
two  aquatic  orders  of  Sirenia  and  Cetacea,  also  in  Manis,  in  a 
sloth  (Chol&pus),  and  an  insectivore  (Chrysochloris).  They 
are  usually  found  on  the  palmar  and  plantar  surfaces,  where, 
in  man  and  the  monkeys,  their  openings  are  readily  seen  at 
regular  intervals  along  the  middle  line  of  the  friction  ridges. 
In  this  location,  by  moistening  the  ridges,  they  undoubtedly 
assist  the  firmness  of  the  grasp,  often  a  factor  of  vital  im- 
portance in  an  arboreal  animal.  When  distributed  over  the 
general  surface  and  yielding  the  customary  colorless  watery 


THE   INTEGUMENT   AND   THE   EXOSKELETON      113 

fluid,  these  glands  are  usually  called  sweat  glands  (glandular 
sudoriparc?),  but  enough  has  been  said  to  show  that  this  term 
is  too  limited  to  be  employed  in  general.  In  Man,  where  they 
fulfill  this  function,  they  do  not  appear  to  be  evenly  distributed, 
and  there  seem  to  be  individual  differences  in  the  extent  and 
copiousness  of  the  secretion.  As  it  has  been  found  that  these 
glands  rarely  occur  in  the  integument  of  the  Fuegians,  there 
are  undoubtedly  marked  racial  differences,  but  there  is,  a,t 
present,  little  knowledge  upon  this  point. 

The  primary  use  of  the  second,  or  acinous,  type  of  integu- 
mental  gland  seems  to  be  to  furnish  an  oily  secretion  for  the 
lubrication  of  the  hair,  forming  the  sebaceous  glands;  and  cor- 
responding to  this  use,  they  appear  less  inclined  than  does  the 
other  type  to  become  disassociated  from  a  hair  follicle. 
Modified  forms  do  occur,  however,  often  unconnected  with 
hair  follicles,  and  modified  in  their  secretion  to  subserve  some 
special  use.  Such  are  the  tar  sal  [meibomian]  glands  of  the 
eyelid,  which  are  properly  the  hypertrophied  sebaceous  glands 
of  the  eyelashes,  whose  purpose  it  is  to  supply  a  line  of  oil  for 
the  edges  of  the  lids  and  thus  prevent  the  overflow  of  tears. 
Other  modified  forms  of  this  type  are  found  at  the  orifices  of 
the  body,  as  the  lips,  the  anus,  and  upon  the  external  genitals 
(Tyson's  glands,  preputial  glands,  etc.).  Corresponding  to 
their  primary  function  as  sebaceous  glands,  they  are  wanting 
in  Cetacea  and  in  adult  Sirenia ;  the  scale-covered  Mqnis  retains 
only  the  modified  orificial  glands.  They  are,  however,  wholly 
wanting  in  some  sloths  (Cholcepus)  and  in  an  African  insec- 
tivore  (Chrysochloris),  the  same  animals  in  which  the'glands 
of  the  tubular  type  are  also  wanting. 

Aside  from  these  small,  generally  distributed,  glands,  the 
integument  of  mammals  is  especially  characterized  by  the  oc- 
currence of  localized  masses  of  glands,  often  voluminous  in 
size  and  furnishing  a  secretion  intended  for  a  special  purpose. 
The  elements  of  which  these  masses  are  composed  are  some- 
times of  the  tubular  and  sometimes  of  the  acinous  type,  or  of 
both  sorts  together.  Of  these  the  anal  sacs  are  widely  dis- 
tributed, composed  mainly  of  tubular  glands,  and  forming 


ii4  HISTORY    OF    THE    HUMAN    BODY 

from  two  to  five  sacs  which  open  into  the  rectum  immediately 
within  the  anus,  from  which  in  some  cases  they  may  be  pro- 
truded by  being  turned  inside  out.  The  best  known  of  these 
are  the  two  lateral  ones  of  the  skunk,  weasel,  and  allied  forms, 
which  are  covered  with  a  muscular  layer  derived  from  the 
levator  ani  muscle,  and  secrete  an  ill-smelling  fluid  as  defense. 
Other  similar  glands  secrete  odoriferous  fluids  employed  as  a 
mode  of  sexual  attraction,  some  of  which  are  agreeable  to  man 
and  are  used  in  the  manufacture  of  perfumes  (musk,  civet). 

In  their  simplest  form  such  glands  open  separately,  but  near 
together,  the  surface  covered  by  the  opening  being  designated 
as  a  glandular  area,  usually  free  from  hair  or  nearly  so ;  but  in 
many  cases  this  glandular  area  becomes  depressed  and  forms  a 
sac  or  bursa  sunken  beneath  the  surface  and  serving  at  times 
as  a  reservoir  for  the  secretion.  It  is  as  such  a  structure  that 
the  mammary  or  milk  glands,  so  characteristic  of  the  Class 
of  Mammalia,  have  arisen,  and  their  appearance  in  the  momo- 
treme,  Echidna,  exhibits  nearly  their  original  condition.  The 
female  of  this  animal  possesses  upon  the  ventral  side  an  integu- 
mental  pouch,  the  marsupium,  in  which  the  eggs  are  placed, 
and  in  which  the  young  are  nurtured  when  hatched.  Opening 
into  the  sides  of  this  pouch  is  a  pair  of  glandular  sacs  or 
pouches,  supplied  with  glands  which  are  probably  of  the  tubu- 
lar type,  although  long  supposed  to  be  acinous.  These  sacs 
are  the  mammary  pockets,  at  the  bottom  of  which  lies  the 
glandular  area  with  its  numerous  openings.  The  secretion, 
which  is  a  form  of  milk,  pours  out  into  the  pockets,  where  it 
is  taken  up  by  the  young.  There  are  no  traces  of  nipples,  but 
the  lips  of  the  sac,  the  corium  wall,  fit  around  the  nose  of  the 
young  and  prevent  loss  (Fig.  31,  a).  A  slight  advance  is 
seen  in  the  young  Halmaturus,  a  marsupial,  where  the 
mammary  pocket  is  deeper,  and  a  rudimentary  nipple  is 
formed  by  the  elevation  of  the  middle  of  the  glandular 
area  at  the  bottom  (d).  This  structure  is  still  further 
developed  in  the  young  opossum  (e),  and  in  this  latter  animal 
the  functional  activity  of  these  organs  causes  the  complete 
extrusion  of  the  nipples,  and  the  mammary  pocket  is  lost  (f). 


THE    INTEGUMENT   AND    THE    EXOSKELETON      115 

This  type  of  nipple,  in  reality  a  mammary  pocket  turned  inside 
out,  is  the  most  usual  among  the  higher  animals.  In  it  the 
glandular  area  forms  the  nipple  itself,  while  the  corium  wall, 
the  rampart-like  lip  of  the  pocket,  becomes  the  areola,  a  circular 


FIG.  31.  Morphology  of  nipples.     [After  WEBER.] 


(a)  Primary  condition,  as  in  Echidna.  (b)  Embryo  calf,  comparable  with  the 
condition  seen  in  (a).  (c)  Cow  (adult)  showing  "  false "  nipple  produced  by  the 
prolongation  of  the  cutis  wall,  (d)  Halmaturus  (a  marsupial)  previous  to  lactation, 
(e)  Didelphys  (a  marsupial)  previous  to  lactation,  (f)  Didelphys  during  lactation, 
showing  "  true  "  nipple,  produced  by  the  eversion  of  the  mammary  pocket. 

In  all  th'e  figures  the  area  glandularis,  *.  e.,  the  surface  of  the  'mammary  pocket, 
is  represented  by  a  dotted  line;  the  cutis  wall  by  a  full  line.  The  branching  lines 
opening  either  at  the  bottom  of  the  depression  or  at  the  summit  of  the  elevation,  are 
the  milk  gland*. 

area  of  modified  skin  surrounding  the  nipple.  Quite  another 
type  of  nipple,  though  derived  equally  with  the  former  from 
the  mammary  pocket  of  the  Echidna,  is  that  occurring  in 
ruminants.  In  this  the  glandular  area  remains  at  the  bottom 
of  the  pocket,  while  the  surrounding  corium  wall  becomes  ele- 
vated more  and  more  until  a  long  pendulous  nipple  is  formed 
from  that  (Fig.  31,  b  and  c).  The  mammary  pocket  is 
here  retained  as  a  long  lactiferous  duct  running  through  the 
nipple.  In  many  placental  mammals  the  earliest  embryonic 
indication  of  the  mammary  glands  consists  of  a  lateral  ridge, 


n6  HISTORY    OF    THE    HUMAN    BODY 

extending  from  axilla  to  groin,  occupying  the  position  of  a 
future  row  of  nipples.  By  the  suppression  of  this  mammary 
ridge  at  regular  intervals  there  arises  a  series  of  elevations 
which  at  first  sight  appear  to  be  the  nipples,  but  which  become 
secondarily  reduced  and  eventually  come  to  form  actual  de- 
pressions. These  are  evidently  the  ontogenetic  repetitions  of 
the  mammary  pockets,  since  from  these,  after  the  manner 
detailed  above,  the  true  nipples  arise,  faithfully  repeating  the 
stages  shown  in  Fig.  31.  The  mammary  ridge  perhaps  repre- 
sents the  wall  of  the  marsupium  or  pouch,  thus  suggesting 
that  the  placental  mammals  have  been  derived  from  ancestors 
which  possessed  a  marsupium.  The  conclusion  is  not  neces- 
sary, however,  that  these  ancestors  were  the  Didelphia  of  the 
present  day,  but  that  the  common  ancestors  of  both  modern 
types  of  mammals,  marsupial  and  placental,  possessed  this 
organ,  and  that  the  Didelphia  have  retained  it  as  a  functional 
organ,  while  in  the  Monodelphia  but  few  traces  remain.  The 
occurrence  of  a  marsupium  among  the  monotremes,  the  only 
living  representatives  of  the  Prototheria,  points  to  the  same 
thing.  (See  Fig.  8,  and  the  accompanying  text.) 

The  number  and  position  of  the  nipples  vary  much  in  the 
different  groups  of  monodelphic  mammals,  and  furnish  a 
series  of  illustrations  of  adaptation,  both  to  the  habits  of  life 
and  the  number  of  the  young.  In  that  type  which  appears  to 
be  the  most  primitive,  there  is  a  series  of  nipples  arranged  in 
a  lateral  row  upon  either  side  and  extending  from  axilla  to 
groin.  In  this  case,  as  in  pigs,  most  carnivora,  and  many 
rodents,  the  animal  lies  upon  one  side  while  nursing.  By  the 
suppression  of  the  anterior  end  of  this  series  inguinal  mamma 
are  produced,  as  in  ungulates,  which  nurse  their  young  while 
standing  erect.  In  the  Cetacea  the  single  pair  of  inguinal 
nipples  lies  in  the  bottom  of  a  pocket,  not  the  primitive  one  of 
the  Echidna,  but  one  secondarily  developed  to  solve  the  prob- 
lem of  nursing  under  water.  The  lips  of  this  pocket  fit  tightly 
about  the  snout  of  the  young,  which  can  suckle  beneath  the 
surface,  being  at  the  same  time  able  to  breathe  through  the 
nostrils,  which  in  these  animals  have  migrated  backward  from 


THE    INTEGUMENT    AND    THE    EXOSKELETON      117 

their  primary  position.  By  a  suppression  of  the  posterior 
portion  of  this  series,  pectoral  mammce  are  produced,  either 
two  pairs,  as  in  some  lemurs,  or  a  single  pair,  as  in  the  majority 
of  primates,  and  in  bats.  In  the  aquatic  Sirenia,  also,  the 
mammae  are  pectoral.  As  they  bear  but  a  single  young  at 
a  time  and  nurse  it  by  clasping  it  in  the  flippers  while  stand- 
ing upright  in  the  water,  these  animals,  as  suggested  by  the 
name,  are  probably  the  real  origin  of  the  well-nigh  universal 
mermaid  myth.  The  pectoral  position  is  the  most  convenient 
for  arboreal  animals  like  the  Anthropoidea  and  enables  them 
to  carry  the  offspring  in  one  arm  and  leave  the  other  free  for 
climbing. 

In  many  animals  with  a  restricted  number  of  mammae  there 
have  been  frequently  observed  cases  of  supernumerary  nipples 
or  supernumerary  mammae.  These  are  termed  respectively 
hyperthelism  and  hypermastism,  and  are  looked  upon  as 
atavistic  and  indicative  of  the  former  development  of  a  com- 
plete series,  of  which  those  normally  developed  form  a  part. 
They  are  often  noted  in  the  adult  human  subject,  and  the 
anlagen  of  numerous  pairs  of  nipples  occur  regularly  in  the 
embryo.  The  occurrence  of  six-nippled  sheep  that  have  a 
tendency  to  cast  two  young  at  a  birth  has  been  recently  made 
the  subject  of  experiment  with  a  view  to  perpetuating  the  latter 
peculiarities,  and  thus  form  a  race  adapted  to  countries  with 
a  short  summer,  like  Canada.  Whether  there  is  a  definite 
correlation  between  these  two  characters,  or  whether  in  Man 
there  is  any  correspondence  between  hypermastism  and  a 
tendency  to  produce  twins,  has  never  been  determined. 

The  occasional  occurrence  of  mammae  in  unusual  positions, 
as  on  the  thigh  or  the  back,  as  has  been  noted  in  the  human 
subject,  is  a  displacement,  and  not  a  reversion,  and  hence  has 
no  normal  morphological  meaning. 

The  occurrence  of  rudimentary  nipples  in  the  male  is  the 
rule  among  placental  mammals,  but  seems  not  to  be  the  case  in 
monotremes  and  marsupials.  If  this  be  true,  this  is  a  definite 
instance  of  the  hereditary  transmission  to  the  male  sex  of 
parts  that  developed  first  in  the  female,  and  formed  for  a  long 


n8  HISTORY    OF   THE    HUMAN    BODY 

time  an  exclusive  characteristic.  Since  neither  natural  selec- 
tion nor  sexual  selection  could  have  a  part  in  this  transmis- 
sion, it  has  been  brought  forward  as  a  case  of  the  direct  trans- 
mission of  an  acquired  characteristic.  It  may,  however,  be 
a  phenomenon  similar  to  the  strange  series  of  homologies 
of  the  various  parts  of  the  reproductive  organs,  treated  in  full 
in  Chapter  IX.,  where  such  an  explanation  is  inadmissable, 
although  at  the  present  time  no  satisfactory  one  can  be  offered. 
Although  usually  the  only  parts  of  the  mammary  apparatus 
to  occur  in  the  male  are  the  rudimentary  nipples,  yet  cases  of 
so-called  gynecomastism  are  known,  in  which  well-defined  and 
even  functional  mammae  occur  in  persons  of  the  male  sex, 
unaccompanied  by  any  sexual  abnormality. 

Pigment  is  a  coloring  matter,  occurring  in  the  form  of 
granules,  and  existing  in  certain  cells  as  a  form  of  metaplasm 
secreted  by  them  and  retained  within  their  substance.  These 
pigment  cells  are  found  in  both  epithelium  and  connective 
tissue.  Pigment  often  occurs  in  the  interior  of  the  body, 
notably  in  the  peritoneum  of  amphibians,  where  it  lines  the 
ccelom  and  invests  the  organs  with  a  brown  or  even  black 
covering.  It  occurs  in  the  integument  or  the  integumental 
structures  of  all  vertebrates  except  certain  white  animals,  and 
in  albinos,  the  peculiarity  of  which  consists  of  a  total  absence 
of  pigment  from  all  parts  of  the  body.  These  two  cases  may 
be  readily  distinguished  by  observing  the  iris  of  the  eye,  which 
retains  its  pigment  in  normally  white  animals,  but  lacks  it  in 
albinos,  giving  the  eyes  a  pinkish  cast.  As  a  general  rule 
the  integument  of  vertebrates  is  pigmented  when  without  ac- 
cessory structures,  or  when  these  form  an  insufficient  covering, 
but  in  those  birds  and  mammals  in  which  the  feathers  or  hair 
are  respectively  sufficient  to  entirely  conceal  the  skin,  these  ac- 
cessory parts  receive  the  color  and  the  integument  is  unpig- 
mented.  This  rule  is  further  emphasized  by  the  fact  that  bare 
places,  like  the  head  and  neck  of  vultures  and  the  ischial  cal- 
losities of  monkeys,  or  scantily  covered  places  like  the  entire 
integument  of  elephants  and  rhinoceroses,  are  pigmented,  and 
occasionally  highly  colored. 


THE    INTEGUMENT   AND   THE    EXOSKELETON      119 

The  pigment  of  vertebrate  integument  is  usually  contained 
in  branching  connective  tissue  cells,  produced  in  the  corium, 
but  capable  of  wandering  into  the  epidermis.  In  some  cases, 
as  in  Man  and  monkeys,  the  stratum  germinativum  of  the  epi- 
dermis is  pigmented,  and  varying  degrees  of  this  are  respon- 
sible for  the  great  variety  of  skin  color  in  Man.  Although 
the  connection  is  hard  to  prove,  it  is  a  general  truth  that 
human  races  that  live  in  the  tropics  are  the  darkest  and  that 
the  skin  grows  gradually  paler  in  races  nearing  the  poles, 
there  being  a  correlation  in  this  respect  between  skin  color  and 
hair  color.  As  instances  of  this,  there  may  be  recalled  the  sooty 
blackness  of  the  Sudanese,  and  the  dark  color  of  the  aborigi- 
nes of  India,  which  may  be  compared  with  the  lighter  color 
of  Europeans  and  northern  Asiatics.  More  convincing  cases 
of  this  are  seen  in  representatives  of  the  same  race ;  such  as  is 
shown  by  the  contrast  between  the  Italians  and  Spaniards  on 
the  one  hand  and  the  Scandinavians  on  the  other,  or  between 
the  American  Indians  in  Canada  and  those  in  Mexico.  A 
similar  reduction  in  pigment  is  found  in  people  living  at  high 
altitudes  in  comparison  with  the  same  race  living  in  the  bor- 
dering lowlands.  The  importance  of  these  correlations  has 
been  repeatedly  denied,  and  there  are  numerous  instances  of 
exceptions,  often  conspicuous  ones  like  the  dark  skin  and  hair 
of  the  Eskimo,  and  at  the  present  state  of  our  knowledge  too 
much  cannot  be  urged  on  this  point. 

Another  point  of  interest  lies  in  the  regions  of  the  body  in 
which  the  pigmentation  is  the  densest.  As  a  general  rule  it 
may  be  observed,  not  in  vertebrates  alone  but  in  invertebrates 
as  well,  that  the  darkest  and  most  deeply  colored  parts  are 
those  that  lie  uppermost,  exposed  to  the  light,  while  the  under 
parts  are  lacking  in  pigment.  That  this  bears  no  relation  to 
the  architecture  of  the  body  may  be  seen  in  the  case  of  the 
flounder,  a  fish  that  is  much  compressed  laterally  and  has  the 
habit  of  lying  upon  one  side  at  the  bottom  in  rather  shallow 
water.  This  habitual  lower  side,  which  is  sometimes  the  left, 
sometimes  the  right,  half  of  the  body,  is  entirely  colorless, 
while  the  upper  side  is  marked  with  a  complicated  pattern 


120  HISTORY   OF   THE    HUMAN    BODY 

resembling  the  sandy  or  muddy  bottom  on  which  the  fish 
lies. 

This  principle  is  a  general  one  and  applies  to  the  distribu- 
tion of  pigment,  whether  in  the  integument  itself  or  in  its 
accessory  structures,  a  fact  that  may  be  readily  seen  by  com- 
paring a  form  with  a  naked  skin,  like  a  frog  or  a  porpoise, 
with  one  covered  with  hair.  In  terrestrial  mammals  the  dif- 
ference is  not  always  a  marked  one  and  is  apt  to  be  greater  in 
short-legged  forms  that  creep  close  to  the  ground ;  in  birds, 
which  in  the  majority  of  cases  expose  nearly  the  entire  surface 
to  the  light,  the  body  may  apparently  be  of  uniform  color,  but 
here  the  deeper  pigmentation  is  confined  to  the  exposed  sur- 
faces of  the  feathers,  while  the  portions  which  are  shielded 
from  the  sun  are  less  deeply  colored  or  even  without  pigment. 
The  white  ventral  surface  of  aquatic  birds  like  snipe  and  gulls 
is  a  protective  coloring  and  comes  under  another  principle. 
In  man  the  distribution  of  pigment  is  also  unequal,  the  darker 
areas  being,  in  all  races,  the  back  and  the  dorsal  aspect  of  arms 
and  legs,  while  the  chest  and  abdomen,  the  ventral  aspect  of 
the  limbs,  and  especially  the  palms  and  soles,  are  lighter  in 
color.  This  distribution,  it  will  be  noticed,  corresponds  to 
the  influence  of  the  light,  when  man  assumes  the  quadrupedal, 
and  not  the  usual  human,  position.  Aside  from  these  general 
areas,  a  deep  local  pigmentation  occurs  in  the  axilla  and  groin, 
about  the  anus  and  the  external  genitals,  and  upon  the  nip- 
ples and  areola,  where  it  is  evident  that  the  pigmentation  is 
for  some  other  purpose  and  can  bear  no  relation  to  the  distribu- 
tion of  light.  In  this  general  connection  between  a  darker 
color  and  the  increase  of  the  sun's  light  and  heat,  there  must 
be  some  physiological  advantage  which  a  dense  pigmentation 
gives  its  possessor,  an  advantage  which  seems  to  be  a  real  one 
whenever  there  is  a  chance  for  comparison  between  a  black 
man  and  a  white  man  in  the  tropics  in  regard  to  their  relative 
power  of  enduring  the  heat  of  the  sun.  Although  the  subject 
is  still  an  obscure  one,  it  seems  probable  that  the  presence  of  a 
dense  layer  of  pigment  in  the  stratum  germinativum  effectually 
prevents  the  direct  action  of  the  light  upon  the  surface  capil- 


THE   INTEGUMENT   AND   THE   EXOSKELETON      121 

laries,  thus  allowing  them  to  expand  and  retain  the  blood  at 
the  surface  where  the  excess  of  heat  can  be  constantly  thrown 
off.  Thus  repeated  observations  have  been  made  in  Samoa* 
when  whites  and  natives  have  been  together  and  doing  the 
same  work,  that  the  skin  of  the  latter  would  be  dry  and  glow- 
ing as  in  a  fever,  while  that  of  the  former  was  cold  and  damp. 
Under  these  circumstances  the  Samoans  would  be  constantly 
giving  off  heat  while  the  whites  were  compelled  to  retain  theirs. 
In  the  presentation  of  the  above  facts  in  connection  with 
one  another,  the  conclusion  seems  almost  unavoidable  that  the 
various  conditions  are  directly  due  to  the  solar  action  upon 
each  individual,  and  to  the  propagation  of  the  conditions  thus 
acquired  until  the  physiological  advantages  become  inborn  in 
the  race.  Although  this  may  seem  at  first  the  simpler  expla- 
nation, there  are  numerous  biological  facts  associated  with 
heredity  that  seem  to  render  impossible  so  direct  a  transmis- 
sion of  somatic  peculiarities,  and  point  to  a  more  indirect 
method  of  attaining  the  same  end  through  individual  variation 
and  the  selection  for  survival  in  each  generation  of  those  in 
which  the  desired  peculiarity  is  the  most  marked.  This  ex- 
planation, however,  is  in  many  points  as  unsatisfactory  as  the 
other  when  applied  to  this  case,  since  we  know  that  the  strug- 
gle for  existence  in  man  has  never  been  severe  enough  to 
compel  the  extinction  of  individuals  differing  from  others  by 
a  shade  of  color;  neither  is  sexual  selection  operative  here, 
since  among  a  primitive  people  all  who  are  not  physically  unfit 
become  the  propagators  of  the  race.  The  matter  must  be  left 
at  present  as  one  in  which  the  facts  are  evident  but  the  explana- 
tion of  them  obscure;  the  problem  is  to  be  solved  sometime, 
and  when  solved  will  offer  an  explanation  of  the  relation  of 
structure  to  environment  everywhere. 

*  According  to   Dr.   A.    Kramer.— Die    Samoa   Inseln,    Stuttgart,    1903. 


CHAPTER   V 
THE   ENDOSKELETON 

".  .  .  our  '  physic  '  and  '  anatomy '  have  embraced 
such  infinite  varieties  of  being,  have  laid  open  such 
new  worlds  in  time  and  space,  have  grappled,  not 
unsuccessfully,  with  such  complex  problems,  that  the 
eyes  of  Vesalius  and  of  Harvey  might  be  dazzled  by 
the  sight  of  the  tree  that  has  grown  out  of  their  grain 
of  mustard  seed." 

THOMAS  HENRY  HUXLEY,  in  his  essay:  On  the 
advisableness  of  improving  natural  knowledge. 

AN  endoskeleton  or  internal  framework  for  the  support  of 
the  muscles  and  the  protection  of  the  viscera  is  one  of  the  dis- 
tinguishing characteristics  of  vertebrates,  for  with  the  excep- 
tion of  a  few  sporadic  cases  in  which  internal  skeletal  parts 
occur,  invertebrates  are  without  such  a  system.  The  verte- 
brate endoskeleton  is  a  part  of  the  connective  tissue  system  of 
the  body  and,  in  its  usual  sense,  includes  only  bone  and  cartil- 
age, although  both  developmentally  and  physiologically  the 
associated  ligaments  and  other  connective  tissues  belong  with 
the  former. 

Primarily  the  endoskeleton  consists  of  three  systems, 
originally  distinct  from  one  another,  the  axial,  the  visceral,  and 
the  appendicular.  The  axial__  includes  the  vertebral  column 
and  a  large  part  of  the  skull ;  the  visceral  includes  the  lower 
jaw,  certain  elements  in  and  about  the  upper  jaw,  the  hyoid 
apparatus,  and  the  branchial  or  gill  arches ;  and  the  appendic- 
ular includes  the  shoulder  and  hip  girdles  and  the  skeleton  of 
the  free  limbs. 

Of  these,  the  axial  is  the  oldest,  and  is  represented  in  its 
simplest  form  by  the  notochord,  although  this  organ  soon 
yields  in  functional  importance  to  the  connective  tissue  sheath 
which  enwraps  it,  from  which,  in  higher  forms,  the  main  ele- 

122 


THE    ENDOSKELETON  123 

ments  of  the  vertebrae  are  derived.  The  notochord,  which 
is  endodermic  and  arises  from  the  primitive  alimentary  canal 
in  the  manner  related  above,  is  the  only  portion  of  the  endo- 
skeleton  not  formed  from  the  mesenchyme,  and  is  hence  not  a 


FIG.  32.  Diagram  of  vertebrate,  showing  relation  of  skeleton  to  soft 
parts. 

AXIAL  SKELETON:  tr,  trabecula;  p,  parachordal ;  d,  dermal  bones  of  skull;  nt, 
notochord;  np,  neuropophyses;  hp,  haemapophyses. 

VISCERAL  SKELETON:     m,  mandible;   br,  branchial  arches. 

APPENDICULAR  SKELETON:  ga,  anterior;  d,  its  dermal  element;  gp,  posterior 
girdle;  x,  anterior  free  limb;  y,  posterior  free  limb. 

SOFT  PARTS:  nv,  nervous  system;  hy,  hypophysis;  a,  aorta;  v,  sub-intestinal  vein 
(an  embryonic  organ) ;  int,  intestine. 

connective  tissue ;  but  this  becomes  gradually  replaced  by 
skeletal  elements  formed  from  true  mesenchymatous  tissue,  so 
that  in  the  higher  forms  the  adult  skeleton  is  wholly  from  this 
latter  source.  The  notochord  seems  to  have  been  a  very 
ancient  form  of  endoskeleton,  antedating  that  formed  of  con- 
nective tissue  and  functioning  in  the  immediate  ancestors  of 
the  present-day  vertebrates.  It  is  present  in  what  may  be 
nearly  its  original  condition  in  Amphioxus,  where  it  appears 
as  a  cylindrical  rod,  extending  through  the  longitudinal  axis 
of  the  body  from  end  to  end.  This  rod  furnishes  a  certain 
degree  of  rigidity  and  allows  the  animal  to  maintain  a  fixed 
length,  while  permitting  a  considerable  amount  of  flexibility 
through  its  elasticity.  In  about  the  same  condition  it  appears 
as  a  constant  organ  during  the  early  embryonic  life  of  every 
vertebrate ;  and,  although  it  is  usually  replaced  in  great  measure 
by  mesenchymatous  elements,  yet  in  some  fishes,  even  in  those 


124  HISTORY   OF   THE   HUMAN    BODY 

as  high  as  ganoids,  it  retains  much  of  its  original  appearance 
and  function.  Both  in  Amphioxus  and  in  these  forms,  as  well 
as  in  all  embryos,  it  is  formed  of  a  semi-gelatinous  tissue, 
often  called  pre-cartilage,  and  is  surrounded  by  a  firm  sheath 
of  connective  tissue,  which,  in  those  adult  forms  with  a  per- 
sistent notochord,  supplies  the  necessary  firmness  and  rigidity 
in  which  particular  the  notochord  alone  would  be  inadequate. 
This  sheath  becomes,  in  fact,  of  far  greater  importance  than 
the  notochord,  and  the  cartilaginous  and  osseous  tissues 
formed  from  it  come  to  encroach  more  and  more  upon  the 
yielding  tissue  within,  and  eventually  supply  the  main  elements 
used  in  the  construction  of  the  vertebrae. 

The  first  stage  in  this  advance  is  seen  in  the  lamprey  and 
other  cyclostomes,  where  there  appear  pairs  of  little  cartilages, 
lying  upon  the  side  of  the  notochord  sheath  and  projecting 
upwards  to  protect  the  nerve  cord,  which  lies  along  the  dorsal 
side  of  the  notochord.  These  little  cartilages  are  of  two  kinds. 
The  primary  ones  develop  from  the  edges  of  the  intermuscular 
septa,  and  are  hence  intersegmental,  a  point  which  is  important 
to  remember  in  connection  with  the  relative  position  of  the 
vertebrae  in  higher  forms.  A  secondary  set  alternate  with 
these,  and  form  intercalary  pieces,  protecting  the  intervals 
between  the  first  set. 

A  second  advance  over  the  condition  found  in  Amphioxus 
is  seen  in  the  formation  of  a  head,  which,  since  here  the  noto- 
chord no  longer  extends  to  the  tip  of  the  snout  but  ends  a 
little  behind  the  plane  of  the  eyes,  has  been  supposed  to  be  in 
part  a  transformation  of  the  anterior  end,  and  in  part  a  new 
formation  added  anterior  to  this.  Whether  this  may  be  safely 
assumed  or  not,  the  anterior  termination  of  the  notochord 
forms  in  the  embryo  an  important  topographical  point,  the 
portion  of  the  head  along  the  sides  of  the  notochord  being 
referred  to  ,as  parachordal,  and  that  anterior  to  it  as  pra- 
chordal.  The  hypophysis,  an  organ  lying  in  the  median  line 
and  depending  from  the  lower  surface  of  the  brain,  lies  at  the 
anterior  point  of 'the  notochord,  and  will  thus  serve  to  mark 
the  boundary  between  these  two  portions  of  the  head. 


THE    ENDOSKELETON 


125 


The  next  few  stages  in  the  history  of  these  parts,  lying 
between  the  condition  above  described  and  definite  vertebrae, 
are  still  somewhat  a  matter  of  controversy,  since,  in  the  various 


a 


c 


d 


FIG.  33.  Diagrams  illustrating  a  theory  of  the  development  of  the  ver- 
tebrate. 

(a)  Condition  previous  to  the  formation  of  vertebral  anlagen  (caudal  region). 
The  body  is  divided  into  segments  by  transverse  myocommata  through  which  run  the 
notochord,  the  nerve  cord,  and  the  aorta.  In  the  region  of  the  coelom  the  myocom- 
mata open  ventrally  and  allow  the  alimentary  canal  to  pass.  There  is  no  trace 
of  bone  or  cartilage,  (b)  Later  stage,  in  which  skeletal  bridges  have  formed  along 
the  edges  of  the  myocommata,  both  dorsally  and  ventrally.  (c)  Detail  of  stiil  later 
stage,  in  which  the  sheath  of  the  notochord  has  chrondrified  (or  ossified)  at  the 
points  where  the  bridges  come  in  contact  with  it.  (d)  Completed  vertebrae,  formed 
by  the  fusion  of  the  elements  shown  in  (c). 

types  of  fish,  where  these  stages  should  be  sought,  numerous 
modifications  have  taken  place  which  are  to  be  explained  as 


126  HISTORY   OF   THE    HUMAN    BODY 

special  adaptations,  admirably  fitted  to  the  habits  and  environ- 
ment of  the  various  species,  but  covering  up  the  original  race- 
history  which  we  are  seeking  to  interpret.  While,  then,  one 
cannot  be  dogmatic  about  this  portion  of  the  history,  the  fol- 
lowing seems  to  be  the  most  likely  course  of  development,  and 
its  various  stages  are  to  be  found,  with  some  little  modifica- 
tion, in  living  species. 

It  would  seem,  then,  that  the  primary  pairs  of  neural 
processes,  for  the  purpose  of  better  fulfilling  their  mission  of 
protecting  the  nerve  cord,  became  more  elongated  until  they 
finally  met  in  the  middle  line  above  the  nerve  cord,  thus  form- 
ing a  series  of  intersegmental  neural  arches,  shaped  like  inverted 
Vs;  and  since  the  aorta,  lying  immediately  beneath  the  noto- 
chord,  needed  a  similar  protection,  other  arches  became 
developed  for  this  purpose,  situated  immediately  below  the 
former,  and  with  their  points  directed  downwards. 

When  these  two  systems  of  arches  were  well  established, 
they  would  naturally  seek  to  secure  a  firmer  attachment  to  the 
notochordal  sheath  by  spreading  out  their  bases,  and  as  the 
two  sets  of  arches,  neural  and  hamal,  were  opposite  each 
other,  each  neural  arch  being  associated  with  its  corresponding 
haemal  arch,  the  enlarged  bases  would  grow  together.  To 
support  the  increasing  weight  of  these  parts,  the  notochordal 
sheath  would  then  chondrify  or  ossify  beneath  these  bases  in 
the  form  of  rings,  and  the  fusion  of  three  elements,  a  neural 
and  a  haemal  arch  and  a  notochordal  ring,  would  form  a 
vertebra  of  the  type  found  in  the  ordinary  bony  fish.  The 
condition  just  described,  with  neural  and  haemal  arches  alike, 
is  that  found  in  the  tail  region,  posterior  to  the  visceral  cavity, 
while  in  the  trunk  the  haemal  arches  are  open  and  their  halves 
widely  divergent,  forming  the  ribs,  which  embrace  the  visceral 
cavity.  The  rings,  whicTTare  formed  from  the  notochordal 
sheath,  begin  in  the  center  of  the  future  vertebrae,  and  as  they 
grow,  expand  along  both  edges  until  they  come  in  contact 
with  the  preceding  and  succeeding  ones.  At  the  same  time, 
however,  the  ossification  has  proceeded  inwards  as  well,  re- 
stricting the  notochord,  and  as  the  central  portion  of  the  ring 


THE    ENDOSKELETON  127 

is  the  oldest,  that  part  becomes  the  most  restricted,  often  com- 
pletely severing  the  notochord  at  this  point,  or  intra-verte- 
brally;  while  the  notochord  is  retained  at  practically  its  original 
caliber  at  the  newest  edges  of  the  ring,-  or  inter-vertebrally. 
The  completed  ring,  which  forms  the  body  or  centrum  of  the 
vertebra,  is  cylindrical,  with  concave  ends  like  the  interior  of 
conical  cups.  Such  vertebrae  are  called  amphiccclous  (=both 
ends  hollow),  and  the  hollows  of  the  adjacent  vertebrae  enclose 
masses  of  notochord  in  the  form  of  two  cones,  placed  base  to 
base. 

Although  the  above  sketch  is,  in  a  way,  hypothetical,  many 
of  the  stages  described  actually  occur  as  the  adult  condition  in 
various  fishes,  especially  ganoids,  and  the  final  condition  is 
exactly  shown  by  the  teleosts,  as  one  may  have  frequent  occa- 
sion to  observe.  That  the  evolution  of  the  vertebral  column 
up  to  this  point  has  been  somewhat  after  the  plan  here  given, 
may  be  generally  conceded,  and  it  is  hoped  that  important 
links  in  the  history  may  be  discovered  in  the  field  of  palaeontol- 
ogy, which  has  already  furnished  us  so  many  valuable  records 
and  bridged  so  many  gaps. 

Above  the  fish  the  development  of  the  vertebral  column  has 
been,  not  so  much  in  the  acquirement  of  new  elements,  as  in 
the  regional  modification  of  those  already  possessed.  This  is 
strikingly  shown  by  the  comparison  of  the  vertebral  column 
of  a  fish  with  that  of  a  reptile  or  mammal ;  in  the  first  of  these 
the  vertebrae  are  all  very  much  alike,  while  in  the  second  they 
are  differentiated  into  successive  groups,  and  in  cases  in  which 
this  differentiation  has  reached  its  extreme  each  vertebra  may 
be  sufficiently  unlike  the  rest  to  be  identified  by  the  anatomist 
when  isolated,  a  feat  impossible  in  the  former  case. 

This  regional  differentiation  is  due  chiefly  though  indirectly 
to  the  change  of  environment  from  water  to  land,  a  change 
which  necessitates  the  replacement  of  soft  and  weak  fins  by 
two  pairs  of  firm  limbs,  and  substitutes  for  the  evenly  dis- 
tributed buoyancy  of  the  water  a  fixed  support  at  two  points, 
the  shoulder  and  hip  girdles.  In  the  first  land  animalsy  the 
limbs  were  small  and  weak,  and  progress  was  attained  trirough 


128  HISTORY   OF   THE    HUMAN    BODY 

a  sinuous  motion  of  the  body.  Even  here,  however,  the  influ- 
ence of  the  limbs  would  be  felt,  since  they  would  cling  to  the 
surface  and  thus  furnish  definite  fixed  points  where  the  sinu- 
ous motion  of  the  vertebrae  would  be  lessened.  By  a  gradual 
increase  in  the  size  and  strength  of  the  limbs,  the  animal  would 
attain  the  power  of  crawling,  that  is,  of  dragging  the  body 
over  the  ground  through  the  action  of  the  limb  muscles  as  well 
as  those  of  the  back,  and  finally  the  limbs  would  become  strong 
enough  to  bear  the  entire  weight  and  the  body  would  be  lifted 
wholly  above  the  surface  of  the  ground,  thus  changing  the 
crawling  motion  into  a  true  walk.  This  gradual  development 
of  the  free  limbs  is  accompanied  by  important  correlated 
changes  in  the  vertebral  column,  due  in  the  main  to  two  causes. 
The  first  of  these  is  the  increased  size  and  functional  impor- 
tance of  the  limb  girdles,  or  those  parts  of  the  limb  skeleton 
enclosed  within  the  body,  to  which  the  free  part  is  movably 
attached,  usually  by  a  ball-and-socket  joint;  and  the  second  is" 
the  increase  in  size  of  the  proximal  limb  muscles.  As  the 
limbs  become  larger  and  stronger,  their  girdles,  i.  e.}  the  proxi- 
mal portion  of  their  skeleton,  feel  the  need  of  a  stronger  sup- 
port and  a  more  intimate  association  with  the  axial  skeleton, 
a  need  especially  felt  by  the  hip-girdle,  since  here  the  greater 
weight  is  sustained.  This  girdle,  seeking  the  necessary  sup- 
port, grows  dorsally  around  the  body,  until  it  meets  the  ends 
of  a  pair  of  ribs  with  which  it  articulates.  In  the  lower 
forms  the  ribs  are  very  short  and  are  borne  upon  the  end  of 
short  transverse  processes,  and  the  girdle  with  its  dorsally 
developing  process,  known  as  the  ilium,  the  rib,  the  transverse 
process,  and  the  vertebra,  form  a  complete  chain  around  the 
body. 

At  first  this  association  involves  but  a  single  vertebra,  the 
location  of  which  is  apt  to  vary.  Thus  in  Necturus]  the  most 
primitive  salamander  now  in  existence,  the  hip-girdle  is  usually 
attached  to  the  iQth  vertebra,  counting  from  the  head,  but  the 
2Oth  is  occasionally  employed  instead,  and  two  cases  have  been 
reported  in  which  the  attachment  was  to  the  i8th.  Cases  are 
also  known  in  which  the  attachment  is  oblique,  either  to  the 


THE    ENDOSKELETON 


129 


on  one  side  and  the  2Oth  on  the  other,  or  to  the  i8th  on 
one  side  and  the  igth  on  the  other. 

In  these  low  forms  this  sacral  vertebra  shows  no  special 
modification  save  that  it  may  be  slightly  stouter  than  its  fel- 
lows, and  have  longer  transverse  processes  and  stouter  ribs, 
but  as  the  posterior  limbs  increase  in  size  and  functional  power 
their  girdle  increases  with  them  and  may  form  similar  attach- 
ment to  two  or  more  adjacent  vertebrae,  which  may  become 
more  and  more  modified  and  form  a  more  or  less  complete 


FIG.  34.  Variations  in  the  composition  of  the  human  sacrum.  [After 
GEGENBAUR.] 

fusion  into  a  single  piece,  the  sacrum.  This  anchylosis  of 
adjacent  sacral  vertebrae  is  the  most  complete  in  birds  and  in 
Man,  and  for  the  same  reason,  namely,  the  employment  of 
the  hind  limbs  alone  for  the  support  of  the  body ;  although  in 
the  two  cases  the  number  and  arrangement  of  the  associated 
parts  differs  very  considerably. 

Variation  in  the  sacral  region  is  not  confined  to  the  lower 
forms,  although  it  is  more  frequent  in  these  latter  (e.  g., 
Necturus)  and  becomes  relatively  stable  in  the  higher  and 
more  specialized  classes.  As  shown  in  Fig.  34,  there  is  varia- 
tion both  in  the  point  of  attachment  of  the  hip  bones  and  in 
the  number  of  vertebrae  involved  in  the  composition  of  the 
human  sacrum,  and  similar  variations  have  been  noted  in  other 
mammals.  These,  like  the  variation  in  the  number  of  ribs  and 
in  the  groups  of  vertebrae,  not  infrequent  in  the  human  subject, 
should  serve  to  dispel  the  idea  that  the  body  of  man  or  any 


130  HISTORY   OF   THE    HUMAN    BODY 

other  animal  is  formed  in  accordance  with  a  definite  pattern 
or  is  constructed  upon  any  other  principle  save  those  of  hered- 
ity and  environment. 

The  anterior,  or  pectoral,  girdle  never  becomes  directly  at- 
tached to  the  vertebral  column,  and  consequently  the  latter 
receives  no  direct  modification  through  the  development  of  the 
former,  but  the  use  of  this  region  as  a  secondary  center  of 
support  causes  a  division  of  function  between  the  vertebrae 
that  lie  anterior  and  posterior  to  it^the  first  forming  the  neck. 
By  the  establishment  of  this  point  and  the  sacrum,  the  two 
centers  of  support,  the  vertebral  column  becomes  divided  into 
regions,  the  differentiation  of  which  depends  upon  the  degree 
of  development  of  the  limbs  and  the  amount  of  difference  in 
the  function  to  which  these  parts  are  subjected.  Beginning 
anteriorly  the  vertebrae  anterior  to  the  first  center  of  support 
are  the  cervical  or  neck  vertebrae,  the  first  one  or  two  of  which 
are  especially  modified  to  bear  the  head  and  allow  of  its  special 
motions.  The  vertebrae  between  the  shoulder-girdle  and  the 
sacrum  are  spoken  of  in  general  as  the  trunk  vertebra,  and  in 
birds  and  mammals  allow  a  further  subdivision  into  thoracic 
and  lumbar,  the  former  being  provided  with  free  ribs,  and  the 
latter  being  without  them.  Then  follow  the  sacral  vertebra, 
usually  more  than  one  in  forms  above  the  amphibia,  followed 
by  the  caudal  vertebra  or  tail.  The  correlation  between  the 
regional  differentiation  and  the  development  of  the  limbs  is 
especially  emphasized  by  such  forms  as  the  whales,  which  have 
secondarily  lost  the  hind  limbs,  and  snakes,  which  have  lost 
both  pairs,  since  in  the  former  the  deprived  region,  and  in  the 
latter  the  entire  vertebral  column,  have  lost  all  trace  of  such 
differentiation. 

The  second  cause  of  modification  of  the  vertebral  column  is 
correlated  with  the  first  and  is  directly  due  to  the  increase  in 
size  of  the  limb  muscles  and  their  consequent  need  of  broader 
and  stronger  points  of  origin.  The  limb  muscles,  in  the  case 
of  animals  with  well-developed  limbs,  are  usually  broad,  fan- 
shaped  sheets,  like  the  trapezius  and  latissimus  dorsi,  attached 
wholly  or  in  part  to  certain  processes  of  the  vertebral  column, 


THE    ENDOSKELETON  131 

and  cause  much  local  differentiation  in  the  varying  degrees  of 
development  of  these  processes.  Although  topographically 
related  to  the  trunk,  and  classed  with  trunk  muscles  in  works 
on  anatomy,  they  belong  morphologically  to  the  limbs,  and 
when  these  latter  are  small,  as  in  salamanders,  the  muscles  are 
small  also,  seldom  extending  to  the  vertebral  column,  and 
thus  exercise  little  or  no  modifying  influence  upon  it. 

Other  modifications  of  the  vertebral  column  are  due  to 
the  movement  of  the  body  as  a  whole  and  to  the  separate  and 
more  or  less  specialized  motions  of  the  head  and  tail.  Thus, 
to  perform  the  crawling  movement  of  salamanders  and  most 
reptiles,  where  a  sinuous  motion  of  the  body  axis  forms  the 
principal  mode  of  locomotion,  there  must  be  a  large  amount 
of  flexibility,  especially  in  regard  to  lateral  movements,  be- 
tween the  separate  vertebrae;  and  thus  the  amphiccelous  form 
of  intervertebral  articulation,  the  restricted  motion  of  which 
proves  sufficient  for  fishes,  becomes  converted  into  true  ball- 
and-socket  joints  by  the  ossification  (or  chondrification)  of 
the  ball  of  notochord  contained  in  the  cavities  between  each 
pair  of  adjacent  cups,  and  by  its  anchylosis  to  one  of  the  con- 
tiguous vertebrae.  This  forms  the  ball;  the  unmodified  cup- 
shaped  end  of  the  other  vertebra  serves  as  a  socket.  The 
anchylosis  of  the  notochordal  balls  may  take  place  with  either 
the  preceding  or  the  succeeding  vertebra ;  in  the  former  case 
each  vertebra  of  the  series  will  have  the  cup  at  its  anterior, 
and  the  ball  at  its  posterior  end,  forming  the  type  known  as 
precocious,  while  in  the  latter  case  the  reverse  condition  is  the 
result,  such  vertebrae  being  designated  as  opisthoc&lous. 

Both  of  these  conditions  are  common  among  amphibians 
and  reptiles,  but  with  the  attainment  of  limbs  sufficiently  stout 
to  entirely  sustain  the  weight  of  the  body  such  a  flexibility  of 
the  vertebral  column  is  not  only  unnecessary  but  becomes  a 
positive  detriment,  and  thus  in  mammals  the  vertebrae  become 
accelous,  that  is,  the  articulations  are  reduced  to  mere  flat 
contact  surfaces,  and  the  notochordal  balls  are  transformed 
into  the  intervertebral  cartilages  that  serve  as  cushions.  In 
the  cervical  vertebrae  of  many  mammals  the  opisthoccelous 


132  HISTORY   OF   THE    HUMAN    BODY 

type  of  articulation  is  retained.  In  birds  even  this  restricted 
motion  is  undesirable  except  in  neck  and  tail,  owing  to  the,  use 
of  the  entire  body  as  an  air-ship  which  must  be  held  in  a  rigid 
position,  and  the  trunk-vertebrse  ossify  into  two  completely 
anchylosed  pieces,  the  first  including  the  thoracic,  and  the  sec- 
ond the  lumbar,  sacral,  and  a  part  of  the  caudal,  vertebrae. 

The  head  is  responsible  for  many  modifications  of  the  verte- 
bral column,  developed  in  part  in  response  to  the  necessity  of 
turning  it  in  all  directions,  and  in  part  to  the  need  of  lifting 
it  from  the  ground,  or  even  sustaining  it  above  the  level  of  the 
rest  of  the  body.  Like  many  others,  these  problems  are  as- 
sociated with  a  terrestrial  environment  and  are  not  experienced 
by  fishes,  in  which  the  main  endeavor  is  to  retain  the  head  in 
a  rigid  state  as  the  direct  anterior  extension  of  the  body  axis, 
since  a  pliant  head  would  render  a  change  of  direction  while 
swimming  of  almost  momentary  occurrence  and  would  entirely 
forbid  those  quick,  arrow-like  propulsions  upon  which  most 
fishes  depend  for  safety  and  for  the  successful  pursuit  of  their 
prey.  In  the  first  experience  of  a  terrestrial  life,  however,  all 
this  becomes  changed.  The  turning  of  the  head  not  only  gives 
an  increased  power  of  observation,  but  is  necessary  in  attack 
and  defense,  and  thus  the  vertebrae  lying  between  the  skull 
and  the  place  of  support  for  the  anterior  limbs  become  differ- 
entiated to  form  a  cervical  or  neck  region,  the  main  en- 
deavor of  which  is  to  gain  flexibility  and  increase  the  mobility 
of  the  head. 

Although  in  some  of  the  higher  terrestrial' vertebrates  this 
power  is  but  little  used,  in  others  it  develops  to  an  extraordi- 
nary extent,  notably  among  the  birds,  in  which  this  is  the 
only  part  of  the  vertebral  column,  except  the  tail,  to  which 
motion  is  allowed.  Here,  in  some  instances,  the  neck  not  only 
becomes  extremely  flexible,  but  greatly  elongated,  accompanied 
by  extraordinary  modifications  of  the  trachea  and  the  blood- 
vessels, in  order  to  accommodate  themselves  to  the  rapid 
changes  of  shape  and  position  of  which  the  neck  becomes 
capable. 

On  the  other  hand,  certain  mammals,  like  the  whales  and 


THE   ENDOSKELETON  133 

porpoises,  and  the  dugongs  or  sea-cows,  which,  having  de- 
scended from  terrestrial  ancestors,  have  become,  secondarily 
adapted  to  an  aquatic  life,  form  a  remarkable  corroboration 
of  the  statement  that  a  movable  neck  is  incompatible  with  a 
natatory  habit,  since  in  these  the  seven  cervical  vertebrae,  typi- 
cal of  the  Mammalia,  have  become  greatly  flattened  antero- 
posteriorly,  and  are  either  fitted  so  closely  together  that  but 
little  motion  is  possible  between  them,  or  are  even  anchylosed 
into  a  single  piece,  thus  not  only  reducing  the  length  of  the 
neck  region  and  approximating  the  head  to  the  shoulders, 
but  depriving  it'  of  motion,  two  important  piscine  charac- 
teristics. 

Not  only  are  the  intervertebral  articulations  in  the  cervical 
region  extremely  flexible  in  general,  but  that  of  the  first  with 
the  skull  and  the  first  with  the  second  become  especially  modi- 
fied, changes  which  often  profoundly  affect  the  shape  of  these 
vertebrae.  The  first  of  these  articulations  is  a  double  modified 
ball-and-socket  joint,  the  protuberances,  or  occipital  condyles, 
occurring  upon  the  occipital  region  of  the  skull  along  the 
lateral  edges  of  the  foramen  magnum,  and  fitting  into  saucer- 
shaped  depressions  on  the  anterior  face  of  the  first  vertebra, 
or  atlas.  In  Amphibia  and  Mammalia  these  condyles  are  wide 
apart  and  distant  from  one  another,  while  in  reptiles  and  birds 
they  coalesce  in  the  mid-ventral  line  and  form  what  appears 
to  be  a  single  median  condyle,  the  two  components  being 
usually  indicated  by  a  median  groove.  In  all  cases  the  motion 
between  the  skull  and  the  atlas  is  in  one  plane  only  and 
imparts  to  the  head  the  bowing  motion. 

The  turning  from  side  to  side  is  effected  by  the  articulation 
of  the  first  vertebra  with  the  second,  and  is  due  to  a  curious 
modification  by  which  the  body  of  the  first  vertebra  remains 
disconnected  from  its  own  neural  arch  and  anchyloses  with 
that  of  the  second  vertebra,  the  axis,  forming  its  pivot-shaped 
odontoid  process,  around  which  the  ring-shaped  atlas  may 
rotate.  This  typical  relation  of  the  first  two  vertebrae,  occur- 
ring in  reptiles,  birds  and  mammals,  is  modified  in  amphibians 
through  a  secondary  inclusion  of  the  elements  of  the  atlas 


134  HISTORY   OF   THE    HUMAN    BODY 

within  the  skull,  leaving  the  axis  with  its  pivot  to  serve  as 
the  first  free  vertebra.  This  bone  secondarily  acquires  artic- 
ular surfaces  to  articulate  with  the  lateral  condyles. 

In  animals  whose  limbs  are  strong  enough  to  sustain  the 
body  above  the  ground  the  weight  of  the  head  and  the 
-necessity  of  holding  it  in  place  beyond  the  anterior  center 
of  support  causes  considerable  modification  of  the  vertebrae, 
the  influences  sometimes  reaching  beyond  the  middle  of  the 
body.  In  these  cases  the  head  is  held  up  in  part  by  muscles, 
but  in  mammals  there  is  also  an  important  auxiliary  appa- 
ratus in  the  form  of  a  strong  ligament,  the  ligamentum 
nuchce,  which  extends  between  the  occipital  region  of  the  skull 
and  the  spinous  processes  of  the  cervical  and  dorsal  vertebrae 
on  a  principle  similiar  to  that  of  a  check  rein. 

In  mammals  with  large  and  heavy  heads,  especially  when 
the  weight  is  augmented  by  voluminous  horns  or  large  tusks, 
the  weight  sustained  by  this  ligament  becomes  enormous,  and 
not  only  is  the  ligament  developed  in  proportion,  but  so,  also, 
are  the  occipital  crests  and  the  spinal  processes  of  the 
vertebrae,  which  serve  it  as  points  of  attachment,  the  pro- 
cesses especially  involved  being  those  of  the  anterior  dorsal 
region  opposite  the  shoulders.  This  correlation  between  a 
heavy  head  and  exaggerated  spinous  processes  is  such  that 
from  a  slight  indication  of  the  one  in  a  fossil  the  other  may 
be  assumed.  In  the  Cetacea,  which  have  the  buoyancy  of 
the  water  to  assist  them,  and  in  Man,  through  the  erect  po- 
sition of  whom  the  head  becomes  almost  perfectly  balanced 
upon  the  summit  of  the  vertebral  column,  this  entire  appara- 
tus, including  the  ligament  and  its  points  of  attachment,  be- 
comes much  reduced,  but  from  a  totally  different  cause  in  the 
two  instances. 

The  tail,  or  post-sacral  region  of  the  vertebral  column,  is 
developed  strictly  in  correlation  with  the  needs  of  the  animal 
and  varies  in  development  from  a  voluminous  portion  of  the 
body,  containing  a  large  number  of  vertebrae  and  furnished 
with  metameric  muscles,  to  a  mere  rudiment,  invisible  ex- 


THE   ENDOSKELETON  135 

ternally.  Examples  of  the  former  may  be  seen  in  salamanders 
and  in  many  snakes,  in  which  the  caudal  region,  that  posterior 
to  the  cloacal  orifice,  may  be  even  more  extensive  than  the 
remainder  of  the  body;  the  opposite  condition  appears  in  the 
frog,  where  the  long  caudal  notochord  of  the  tadpole  becomes 
in  the  adult  consolidated  into  an  unsegmented  urostyle,  situ- 
ated between  the  two  elongated  ilia  and  entirely  enclosed  by 
the  soft  parts.  Similar  reductions  are  found  in  birds,  in  which 
the  tail  skeleton  consists  of  six  free  and  six  anchylosed  verte- 
brae, and  in  the  higher  anthropoids,  in  which  the  3-5  embryonic 
vertebrae  become  in  the  adult  consolidated  into  a  single  piece 
(coccyx). 

There  are  two  distinct  sets  of  ribs  developed  among  Ver- 
tebrates, having  a  slightly  different  history,  but  subserving  the 
same  general  purpose,  that  of  protecting  the  viscera,  and  of 
furnishing  attachments  for  the  muscles.  Since  one  set  is  suf- 
ficient for  use  in  the  same  animal,  both  do  not  occur  simul- 
taneously save  in  a  single  instance,  but  the  one  set  is,  with  some 
exceptions,  characteristic  of  fishes,  the  other  of  higher  forms. 
The  origin  of  the  first  of  these  sets  has  been  already  explained 
in  the  discussion  of  vertebras,  where  they  were  described  in 
teleost  fishes  as  expansions  of  the  lower  or  haemal  arches. 
The  other  ribs  have  in  their  origin  no  direct  connection  with 
the  vertebrae,  that  is,  they  are  not  derived  from  portions  of 
them,  .but  develop  from  the  free  edges  of  the  intermuscular 
septa,  the  myocommata,  where  they  border  upon  the  visceral 
cavity.  This  process  of  rib  formation  does  not  necessarily 
involve  the  entire  free  edge  of  the  septa,  but  is  confined  in 
lower  forms  to  the  extreme  dorsal  region,  bordering  on  the 
vertebras,  with  which  they  articulate.  Thus  in  selachians, 
almost  the  only  fish  that  possess  this  sort  of  rib,  and  in  amphib- 
ians, they  are  very  short,  being  functionally  scarcely  more 
than  movable  tips  for  the  transverse  processes  and  of  no  value 
for  the  protection  of  the  viscera.  They  make  no  attempt  to 
reach  around  the  body,  and  thus  never  come  into  relation  with 
a  sternum.  This  latter,  to  us  the  typical  relation  for  ribs  to 
assume,  appears  first  in  reptiles,  which  thus  form  the  prototype 


HISTORY   OF   THE    HUMAN    BODY 


of  the  later  development  in  birds  and  mammals.  In  these 
latter  a  typical  rib  possesses  two  well-marked  segments,  a 
dorsal  and  a  ventral,  often  bent  at  an  angle  to  each  other ;  both 


^  A 


FIG.  35.  Morphology  of  ribs.     [After  WIEDERSHEIM.] 

(a)  Ganoid,  (b)  Dipnoan.  (c)  Teleost.  (d)  Selachian,  (e)  Polypterus  (a  spe- 
cial case  among  ganoids),  (f)  Urodele. 

In  the  three  first  the  condition  in  both  trunk  and  tail  is  given.  In  all  the  figures 
the  "  fish  rib  "  is  striped,  the  myocommatous  rib  is  black,  and  the  basal  stumps  are 
outlined. 

may  be  fully  ossified,  as  in  birds,  or  the  ventral  segments 
may  remain  cartilaginous,  forming  the  so-called  "  costal  car- 
tilages," characteristic  of  mammals.  In  birds  the  dorsal  seg- 
ments possess  flat  uncinate  processes,  which  extend  backwards 
from  their  posterior  edges  and  overlap  the  succeeding  rib,  thus 


THE   ENDOSKELETON  137 

effecting  here  the  rigidity  necessary  in  all  parts  of  the  body  in 
adaptation  to  flight  while  allowing  for  play  of  the  respiratory 
motions. 

The  distribution  of  the  two  types  of  ribs  among  vertebrates 
is  a  little  unusual,  since  the  second  or  myocommatous  type 
appears,  not  only  in  amphibians  and  amniotes,  the  higher 
groups,  but  also  in  the  selachians,  one  of  the  most  primitive. 
This  is  one  of  the  many  indications  of  kinship  between  these 
and  the  higher  forms,  and  suggests  the  direct  descent  of  the 
amphibians  from  selachian-like  ancestors,  thus  disposing  of 
the  remaining  fishes  as  collateral  lines,  in  which  the  piscine 
type  attains  its  special  line  of  development,  without  relation- 
ship to  the  higher  classes,  save  through  a  common  ancestor. 
The  haemal  arch  ribs,  or  true  "  fish-ribs,"  are  characteristic  of 
teleosts,  dipnoi,  and  most  ganoids ;  in  one  of  the  latter,  Polyp- 
terus,  both  sets  appear  simultaneously,  the  myocommatous  set 
being  functional,  while  the  haemal  arch  set  is  rudimentary,  not 
attached  to  the  vertebrae,  and  hence  of  little  use  to  the  fish, 
but  of  great  significance  to  the  anatomical  historian. 

In  a  strict  sense  it  cannot  be  said  that  the  fish-rib  or  haemal 
arch  ribs  are  in  all  cases  exactly  homologous  with  one  another, 
or  even  that  they  are  in  all  cases  formed  mainly  from  the 
haemal  arches,  since  recent  investigation  has  demonstrated  the 
existence  of  other  elements,  derived  directly  from  the  bodies 
of  the  vertebrae,  and  normally  supporting  the  haemal  arches, 
which  are  often  concerned  in  the  formation  of  the  ribs;  but 
not  only  would  an  exposition  of  this  lead  us  too  far  into  details, 
but  would  take  us  away  from  the  main  inquiry,  since  the  phe- 
nomena do  not  occur  on  the  direct  road  traced  in  our  present 
history.  '  The  conception  of  these  ribs  as  expanded  haemal 
arches  is  not  in  any  case  far  from  the  truth,  since  the  other 
elements  concerned  are  themselves  functionally  if  not  mor- 
phologically parts  of  the  haemal  arches. 

As  shown  by  amphibians  and  reptiles  every  vertebra  5e>~ 
tween  the  second    (axis)    and  the  sacrum   is  typically   fur- 
nished with  a  pair  of  ribs,  which  in  these  Classes  are  usually 
free.     In  birds  and  mammals  certain  of  these  become  anchy- 


138  HISTORY    OF    THE    HUMAN    BODY 

losed  to  the  vertebrae  which  bear  them,  leaving  a  set  of 
thoracic  vertebrae  (the  "  dorsal  "  vertebrae  of  the  older  termin- 
ology) interpolated  between  two  groups,  cervical  and  lumbar, 
in  which  the  rib  elements  are  fused.  In  the  cervical  vertebrae 
these  fused  ribs  form  the  ventral  element  of  the  plainly  double 
transverse  processes,  and  enclose  between  themselves  and  the 
original  transverse  process  (diapophysis)  the  vertebral  for- 
amina. In  the  lumbar  vertebrae  the  rib  elements  form  the 
large  wing-like  transverse  processes  (pleurapophyses),  and 
are  thus  seen  to  be  not  equivalent  to  the  processes  of  the  same 
name  in  other  regions. 

The  number,  both  of  free  ribs  and  of  vertebrae  forming 
each  group,  differs  considerably,  not  only  in  different  mam- 
mals, but  even  in  different  individuals  of  the  same  species. 
Thus  in  Man,  although  twelve  pairs  of  free  ribs  is  the  rule, 
the  rib  element  of  the  last  (7th)  cervical  vertebra  is  occasion- 
ally free,  "  cervical  rib,3'  and,  more  commonly,  a  free  rib  ap- 
pears on  the  first  lumbar  vertebra.  As  this  is  perhaps  the 
rule  rather  than  the  exception  in  the  gorilla,  one  of  Man's 
nearest  living  allies,  this  anomaly  is  often  called  the  (f  gorilla 
rib."  Variation  in  the  sacral  vertebrae  has  already -been 
noticed  [Cf.  Fig.  34  and  accompanying  text]. 

The  origin  of  the  sternum  is  still  a  matter  of  controversy, 
and  it  seems  likely  that  there  may  have  been  two  sternums, 
of  different  origin,  the  one  succeeding  the  other  during  his- 
torical development,  the  archisternum  and  the  neosternum. 
Fishes  lack  the  part  entirely,  but  it  is  present  in  some  form 
in  all  other  vertebrates  save  in  a  few  aberrant  cases,  for  ex- 
ample, the  snakes,  where  it  is  incompatible  both  with  their 
mode  of  locomotion  by  means  of  the  ends  of  the  very  numer- 
ous ribs,  and  with  their  habit  of  swallowing  huge  mouthfuls, 
far  too  large  to  pass  through  the  ring  formed  by  the  vertebrae, 
ribs,  and  sternum,  as  is  the  usual  arrangement.  What  is  ap- 
parently the  first  indication  of  a  sternum  is  seen  in  the  sala- 
mander Nee turns ~,  perhaps  the  lowest  amphibian,  in  which 
from  three  to  five  of  the  thoracic  myocommata  chondrify  in 
the  ventral  region,  forming  small  V-shaped  elements,  indefi- 


THE    ENDOSKELETON 


139 


nite  in  shape  and  irregular  in  occurrence,  as  is  usually  found 
in  an  organ  at  its  beginning,  before  the  type  has  become  fixed. 


d 


in — 


c 


FIG.     36.     Morphology   of  the   sternum. 

(a)  Necturus  (a  primitive  salamander),  (b)  A  higher  salamander,  (c)  Frog, 
(d)  Lacerta  (European  lizard),  (e)  Cat. 

c,  coracoid;  d,  epicoracoid;  e,  episternum;  f,  clavicle;  g,  scapula;  h,  suprascapula, 
p,  procoracoid;  st,  sternum;  m,  manubrium;  stb,  sternebrae;  x,  xiphisternum. 

If  we  seek  the  reason  for  their  appearance  we  shall  probably 
find  it  in  an  attempt  to  lessen  the  pressure  upon  an  important 


140  HISTORY    OF    THE    HUMAN    BODY 

point.  One  of  the  sternal  elements,  usually  the  largest,  lies 
in  the  fourth  myocomma,  in  close  connection  with  the  over- 
lapping coracoids,  and  as  at  the  same  point  in  higher  salaman- 
ders there  is  a  definite  sternal  plate  of  a  rhomboid  shape,  this 
latter  has  evidently  developed  from  the  element  in  question, 
while  the  others  have  been  lost.  This  must  also  be  the  same 
piece  found  in  frogs  and  other  tailless  amphibians,  again  in 
the  same  relationship  to  the  coracoids,  and  entering  into  a 
more  or  less  complete  connection  with  the  two  halves  of  the 
shoulder-girdle  in  forming  the  skeletal  armature  that  covers 
the  pectoral  region.  As  the  ribs  of  all  amphibia  are  very  short 
and  rudimentary,  and  do  not  reach  even  half  way  around  the 
body,  there  is  never  the  slightest  attempt  at  a  connection  be- 
tween them  and  the  sternal  piece,  a  characteristic  that  defi- 
nitely distinguishes  this  archisternum  from  the  neosternum 
of  the  Amniota.  This  last  organ,  the  second  form  of 
sternum,  is  characteristic  of  reptiles,  birds,  and  mammals, 
and  is  not  only  always  connected  with  several  pairs  of 
thoracic  ribs,  but  undoubtedly  owes  its  origin  to  them,  being 
probably  due  to  the  fusion  of  the  ribs  in  the  mid-ventral 
line.  This  fusion  forms  in  reptiles  and  birds  a  flat  plate, 
especially  extensive  in  the  latter,  where  it  serves  as  a  place  of 
origin  for  the  enormously  developed  muscles  of  flight,  but  in 
the  mammals  the  sternum,  continuous  with  the  ribs  while  in 
the  cartilaginous  state,  ossifies  in  the  form  of  a  series  of  sepa- 
rate elements,  the  sternebra,  one  for  each  pair  of  ribs  involved. 
The  original  number  of  these  elements  may  be  retained 
throughout  life,  as  in  most  mammals,  or  may  become  reduced 
by  a  secondary  fusion  to  a  smaller  number. 

The  confinement  of  the  sternum  to  the  thoracic  region 
leaves  the  ventral  abdominal  surface  unprotected,  an  affair 
of  no  great  moment  so  long  as  an  animal  remains  small,  or 
not  very  much  elongated,  but  when,  as  in  the  Crocodilia,  the 
elongation  of  the  body  greatly  increases  the  extent  of  the 
unprotected  surface,  while  at  the  same  time  the  increase  in 
size  renders  the  body  ponderous,  the  pressure  exerted  on  the 
abdominal  viscera  by  the  weight  of  the  body  as  the  animal 


THE    ENDOSKELETON  141 

crawls,  or  even  lies  passively  on  the  surface  of  the  ground, 
must  be  very  great.  It  is  evidently  to  overcome  this  in  part 
and  furnish  some  protection  for  the  soft  parts  that  there  de- 
velops in  this  region  a  series  of  skeletal  elements  precisely 
similar  in  origin  to  the  primitive  sternal  pieces  of  Nectums, 
formed  by  the  ossification  of  the  ventral  portion  of  the  ab- 
dominal myocommata.  Developing  along  the  mid-ventral 
line  also,  many  of  the  pieces  become  connected  together  and 
form  a  system  of  "  abdominal  ribs,"  as  they  have  been  called, 
better  known  as  the  parasternum.  As  these  do  not  appear  to 
be  represented  in  any  other  Order,  they  are  of  no  phylogenetic 
value,  but  serve  to  explain  the  reason  for  the  origin  of  the 
archisternum  in  the  salamanders  by  furnishing  an  exact  phy- 
siological parallel.  Associated  with  the  sternal  region,  both 
in  Amphibia  and  in  the  Amniota,  there  is  a  rather  problematic 
element,  known  as  the  episternum,  of  which  no  continuous 
history  is  yet  known,  so  that  it  is  not  even  certain  that  the 
various  elements  in  different  animals  called  by  that  name  are 
homologous.  The  typical  episternum  is  a  skeletal  piece  occur- 
ring in  lizards  and  consisting  of  a  thin  cross-shaped  or  T-- 
shaped piece  lying,  as  its  name  denotes,  upon  (i.  e.,  on  the 
ventral  side  of)  the  sternum,  and  a  little  anterior  to  it. 

This  part  is  not  clearly  present  in  other  vertebrates,  but 
similar  pieces  occur  in  several  cases,  and  are  often  designated 
by  the  same  name.  Thus,  in  the  shoulder-girdle-sternum  com- 
plex of  the  frog  there  is  a  piece  extending  anteriorly  along  the 
mid-ventral  line,  between  the  clavicles,  and  closely  resembling 
the  true  sternum  (archisternum)  that  extends  posteriorly. 
This  has  been  often  called  the  episternum,  but  is  more  likely 
a  portion  of  the  archisternum,  formed  like  the  other,  from 
myocommata.  Again,  the  well-known  "  wish-bone  "  of  birds 
is  formed  by  a  fusion  of  the  two  clavicles  with  a  middle 
piece,  the  inter  clavicle,  which  forms  the  "  head  "  and  is  es- 
pecially well  developed  in  the  common  fowl.  This  element  also 
has  been  identified  with  the  episternum  by  some  investigators, 
as  have  also  certain  parts  of  the  keel  of  the  sternum,  which 
develop  from  separate  centers  of  ossification. 


142 


HISTORY   OF   THE    HUMAN    BODY 


Among  the  Mammalia  the  lowest  Order,  the  Monotremata, 
possess  in  this  region  a  large  T-shaped  bone,  the  stem  of 
which,  very  broad  and  flat,  articulates  with  the  true  sternum, 
forming  its  anterior  extension,  while  the  lateral  arms  are  ap- 
plied along  the  sides  of  the  clavicles.  This  piece  has  been 
called  by  some  an  episternum  and  by  others-  an  interclavicle, 
but  its  precise  homologies  are  not  definitely  determined.  In 
all  other  mammals  the  clavicles  apparently  articulate  directly 
with  the  most  anterior  of  the  sternal  pieces,  the  manubrium; 


FIG.   37.  Sternum    and   shoulder-girdle   of   mammals.      [After   W.    K. 
PARKER.] 

(a)    Ornithorhynchus.      (b)    Human   embryo. 

c,  coracoid;  d,  epicoracoid;   e,  episternum;  f,  clavicle;  g,  scapula;  h,  suprascapula; 
m,  manubrium;   stb,  sternebrae;   x,  xiphisternum. 

but  in  the  embryo  there  are  found  definite  disc-shaped 
skeletal  elements,  interposed  between  the  two,  which  develop 
later  into  thin,  interarticular  discs.  These,  usually  designated 
omosternum,  have  been  likened  to  the  lateral  arms  of  the 
T-shaped  bone  of  the  Monotremata. 
X"The  ontogenetic  history  of  the  skull,  a  complex  of  skeletal 

/  elements  developed  at  the  anterior  end  of  the  notochord,  is 
singularly  constant  in  all  classes,  and  we  may  thus  feel  con- 

\  fident  that  we  have  in  this  a  repetition  of  stages  once  passed 


THE    ENDOSKELETON  143 

through  by  the  adult  ancestors  of  the  present-day  vertebrates. 
It  is  true  that  the  early  stages  thus  indicated  do  not  correspond 
with  the  adult  condition  of  any  form  now  living,  but  of  the  two 
types  in  which  we  might  expect  to  find  a  correspondence 
with  this  period  of  the  history,  Amphioxus  has  no  head,  and, 
of  course,  no  skull,  and  the  cyclostomes  with  their  parasitic 
habit  are  too  much  modified  to  be  reliable ;  there  is,  moreover, 
an  enormous  gap  between  the  t\yo  ancKa  secqrKi,  almost  as 
great,  between  the  latter  and  the  selacmans/sVUiat:  it  may  well 
be  conceded  that  adult  animals  representing  the  stages  indi- 
cated by  the  embryonic  history  once  existed  in  the  places  now 
left  vacant.  Nothing  could  fit  better  into  this  ontogenetic 
history  at  a  later  period  than  the  selachian  skull,  as  will  be 
shown  further  on,  thus  verifying  the  record  at  an  important 
point,  and  rendering  it  more  probable  that  the  earlier  em- 
bryonic stages,  so  constant  in  appearance  in  all  vertebrates, 
are  equally  accurate  in  reproducing  the  conditions  once  found 
in  forms  now  lost  to  us.  To  outline  the  history,  then,  with  the 
help  of  embryology,  it  appears  that  the  ancestral  vertebrate, 
after  the  acquirement  of  the  prachordal  addition  to  its  head, 
developed  several  pairs  of  external  sense  organs  in  the  cephalic 
region,  three  of  which,  the  nasal  sacs,  the  eyes,  and  the  (inner) 
ears,  have  persisted.  Of  others  there  are  indications  in  early 
embryonic  life,  such  as  the  one  placed  between  the  eye  and  ear 
and  supplied  by  the  seventh  nerve,  and  there  are  reasons  to 
believe  that  the  original  sense  organ  of  the  second  pair  was 
not  the  eye  as  we  have  it  now,  but  the  lens  alone,  in  the  form 
of  a  simple  capsule;  but  these  matters  hardly  belong  in  this 
place  and  are  suggested  merely  as  indications  of  the  elaborate 
past  history  of  the  head,  entirely  gone  from  the  world  of 
adult  life,  but  now  restored  in  part  by  the  labors  of  a  gener- 
ation of  embryologists.  At  this  time  the  notochord,  termi- 
nating at  the  hypophysis,  a  downgrowth  of  the  brain  just  an- 
terior to  the  ear  capsules,  was  the  only  skeletal  element  in 
the  head,  and  could  have  had  little  value  as  an  organ  of  sup- 
port, and  none  whatever  as  an  organ  of  protection.  This 
condition  of  affairs  is  represented  in  Fig.  38,  A,  which,  it 


144 


HISTORY   OF   THE    HUMAN    BODY 


must  be  noted,  represents  the  head  as  seen,  not  from  above, 
as  it  is  more  usually  drawn,  but  from  below,  a  view  that  en- 
ables one  to  see  the  notochord  and  its  termination  behind 
the  hypophysis.  To  this  condition  there  are  added,  but  no 
one  yet  knows  how  or  from  what  source,  two  pairs  of  later- 
ally placed  cartilages  (Fig.  38,  B),  the  one  alongside  the  noto- 


FIG.  38.  Diagrams  showing  the  development  of  the  primordial  skull. 
Since  this  organ  develops  primarily  beneath  the  brain  as  a  support  the 
figures  represent  the  ventral  aspect. 

(A)  Early  stage,  before  the  appearance  of  cartilage.  The  notochord  is  seen  lying 
along  the  nerve  cord  as  far  forward  as  the  hypophysis.  The  three  sense-organs, 
nose,  eye,  and  ear,  have  already  appeared.  (B)  This  stage  shows  the  trabeculae  [t], 
the  parachordals  [p],  and  the  capsules  around  the  sense-organs.  (C)  In  this  the 
trabeculae,  the  parachordals,  and  the  nasal  and  otic  capsules  have  fused  into  a  single 
mass,  the  primordial  skull,  or  chondrocranium.  The  anterior  end  of  the  notochord 
is  imbedded  in  this.  The  cartilaginous  capsule  of  the  eye  remains  free  to  allow 
the  necessary  movements  of  the  eyeball. 

chord  and  the  other  anterior  to  .it,  the  parachordal  and  prce- 
chordal  elements  respectively.  The  former  are  rather  flat,  of 
an  elongated  crescentic  shape,  filling  in  the  space  between  the 
notochord  and  the  ear  capsules;  the  latter  are  elongated  and 
rod-like  or  beam-like,  hence  often  termed  the  trabeculce  (di- 
minutive of  trabs,  trabis,  a  beam),  and  lie  a  little  beneath  the 
eyes  and  nearer  the  median  line. 

At  the  same  time  the  three  persisting  pairs  of  sense  organs 


THE   ENDOSKELETON  145 

become  enclosed  by  cartilaginous  capsules,  differing  some- 
what in  their  development,  according  to  the  needs  of  the  organ. 
Thus  the  nasal  capsules  remain  open  anteriorly  for  the  free 
admission  of  the  fluids  to  be  tested,  the  eye-capsules  involve 
the  sclera  alone,  while  the  otic  capsules  usually  become  entirely 
closed  and  develop  fairly  thick  walls,  since  sound  vibrations  can 
pass  easily  through  solids  and  do  not  need  a  special  opening. 
As  these  several  cartilaginous  elements,  the  para-  and  prae- 
chordals  and  the  sense  capsules,  increase  in  size,  they  fuse 
together  in  about  the  following  manner.  The  two  trabeculse 
expand  anteriorly  and  fuse  with  each  other  across  the  middle 
line  and  with  the  nasal  capsules  as  well ;  extending  backwards, 
they  fuse  with  the  parachordals.  These  latter,  growing  in 
width,  fuse  both  with  the  otic  capsules  and  with  each  other, 
including  in  this  fusion  the  anterior  end  of  the  notochord, 
which  becomes  lost  in  the  general  mass. 

There  is  thus  formed  a  single,  curiously  shaped  piece  of 
continuous  cartilage,  composed  of  all  the  elementary  pieces, 
with  the  natural  exception  of  the  otic  capsules,  which  must 
remain  free  to  allow  the  turning  of  the  eyeball  (Fig.  38,  C). 
These  pieces  fuse  so  completely  that  all  boundaries  are  lost, 
and  we  can  speak  only  of  a  parachordal  or  a  trabecular  re- 
gion, and  so  on,  without  assigning  definite  boundaries.  This 
consolidated  piece  is  termed  the  primordial  skull  or  chondro- 
cranium,  and  remains  at  this  stage  in  selachians,  where  it  is 
characteristic  of  the  entire  Order,  being  throughout  life  with- 
out trace  of  ossification  and  with  such  slight  modifications 
only  as  are  necessary  for  the  adaptations  of  the  various  adult 
forms.  It  is  a  natural  supposition  drawn  from  common  ex- 
perience that  a  skull  is  intended  for  the  protection  of  the 
brain,  but  in  this  case  the  function  is  rather  that  of  support, 
since  it  lies  laterally  to  and  in  part  beneath  the  brain,  leaving 
practically  the  entire  dorsal  and  the  anterior  part  of  the  ven- 
tral aspects  without  protection.  In  the  adult  selachians,  in- 
deed, these  deficiencies  are  made  up  in  part  by  the  formation 
of  firm  membranes,  continuous  with  the  cartilage  and  closing 
in  the  open  fontanelles,  but  they  are  plainly  secondary  modifi- 


146  HISTORY   OF   THE    HUMAN    BODY 

cations,  for  use  during  active  adult  life,  and  are  not  empha- 
sized in  the  embryonic  history  of  higher  forms. 

This  stage  of  the  chondro cranium,  or  the  selachian  stage, 
as  it  may  be  called,  is  passed  through  with  during  the  de- 
velopment of  all  the  higher  vertebrates,  and  although  in 
the  various  forms  the  shape  and  proportion  of  the  parts  often 
differ  widely  in  anticipation  of  the  various  needs  of  the  adult, 
they  all  possess  in  common  the  origin  in  the  same'  way,  from 
the  same  elemental  parts,  and  the  characteristic  regions  may  in 
all  cases  be  readily  identified. 

For  the  next  stage  in  this  history  it  will  not  be  necessary 
to  have  recourse  to  embryology  save  to  verify  the  conclu- 
sions, since  it  is  represented  with  almost  diagrammatic  clear- 
ness among  the  ganoids,  a  very  few  of  which  have  been,  by 
a  fortunate  chance,  saved  from  the  general  destruction  of  the 
Order  during  an  earlier  geological  period.  This  stage  may  be 
thus  conveniently  denominated  the  ganoid  stage,  for  the  type 
of  which  we  may  select  the  sturgeon.  Although  similar  to  the 
selachians  in  many  respects,  this  animal  differs  markedly 
from  them  in  its  external  covering,  for  while  the  former  is 
evenly  and  uniformly  covered  by  small  placoid  scales  arranged 
in  a  regularly  imbricated  pattern,  the  sturgeon  possesses  a 
series  of  large,  bony  plates,  or  scutes,  as  they  are  called, 
which  may  be  considered  as  having  been  formed  originally 
from  the  fusion  of  the  basal  pieces  of  many  scales.  These 
scutes  are  arranged  on  the  body  in  longitudinal  rows,  leaving 
the  intervening  regions  bare,  but  are  continued  over  the  head 
as  somewhat  modified  scutes,  the  edges  of  which  are  in  con- 
tact, thus  forming  an  external  armor,  with  sutures  between  the 
different  scutes  (Fig.  19,  A).  Immediately  beneath  this  lies 
a  cartilaginous  skull,  very  similar  to  that  of  selachians,  and 
the  dermal  armor  encases  it  like  an  external  skull,  which  it 
really  is.  These  dermal  plates  are  quite  definite  in  their 
arrangement,  and  the  same  general  plan  may  be  followed 
throughout  the  Order  of  ganoids.  The  snout,  or  rostrum,  is 
covered  by  a  series  of  small  rostral  plates,  which  extend  back 
as  far  as  the  nostrils ;  back  of  these  openings  may  be  found  a 


THE   ENDOSKELETON  147 

pair  of  nasals;  behind  these  again,  and  between  the  eyes,  is  a 
pair  of  frontals,  often  accompanied  by  prce-  and  post-frontals. 
Behind  these  is  a  pair  of  parietals,  and  one  or  more  supra- 
occipitals.  On  the  sides  of  the  head,  at  about  the  level  of  the 
parietals,  are  the  squamosals,  and  around  the  eye  are  several 
orbitals,  distinguished  as  pre-,  supra-,  post-orbitals,  etc.  The 
operculum,  or  gill-flap,  which  is  present  in  these  fishes,  is  cov- 
ered and  augmented  by  supra-,  sub-,  and  pre-operculars. 

In  short,  to  anticipate  the  history  a  little  at  this  point,  we 
see  in  the  dermal  scutes  the  first  appearance  of  the  so-called 
dermal  bones  of  the  skull  which  in  later  forms  are  to  sink  in 
beneath  the  surface  and  become  internal,  thus  coming  into 
close  connection  with  the  primordial  skull  and  the  osseous 
elements  derived  from  it.  They  are  not  all  inherited  by  higher 
forms  exactly  as  they  occur  in  the  ganoid,  the  question  of 
their  retention  being  based  in  each  case  upon  their  functional 
importance.  Thus,  the  opercular  series,  retained  in  the  fish, 
becomes  lost  with  the  reduction  of  the  part  which  they  cover ; 
the  orbital  series  is  retained  in  part  by  reptiles,  but  becomes 
lost  in  birds  and  mammals,  with  the  single  exception  of  one  of 
the  prae-orbitals,  which  becomes  the  lacrimal;  and  the  supra- 
occipital  series  becomes  reduced  to  a  single  piece.  On  the 
other  hand,  certain  ones  are  retained  in  all  higher  vertebrates, 
and  are  recognizable  throughout,  although  by  secondary  fu- 
sions and  divisions  they  are  not  always  strictly  homologous. 
Thus,  the  frontals  may  or  may  not  include  the  originally 
separate  prae-  and  post-frontals,  and  in  a  given  case  the  ab- 
sence of  one  of  these  latter  elements  as  a  distinct  piece  may 
mean  either  that  it  has  fused  with  one  of  the  others  or  has 
been  gradually  reduced  in  size  until  it  has  become  lost.  The 
frontals  in  some  form,  however,  are  among  the  most  constant 
of  dermal  elements,  and  the  same  may  be  said  of  the  parietals, 
squamosals  and  nasals,  which  can  be  traced  in  all  the  verte- 
brate classes  (Fig.  19).  The  ventral  side  of  the  cranium 
becomes  also  encased  in  a  similar  manner  by  dermal  bones  that 
develop  in  the  roof  of  the  mouth,  among  which  are  the  vomers, 
the  palatines,  the  pterygoids,  and  the  extensive  parabasal 


i48  HISTORY   OF    THE    I^UMAN    BODY 

* 
*  > 

(parasphenoid*) ,  which  forms  almost  the  entire  base  of  the 
cranium  in  fishes  and  amphibians.  Certain  of  these  last  named 
do  not  develop,  strictly  speaking,  iivassociation  with  the  cra- 
nium, but  are  formed  about  certain'elements  of  the  visceral 
skeleton,  as  will  be  explained  below,  but  as  these  latter  ele- 
ments early  lose  their  physiological  independence  and  become 
closely  incorporated  with  the  original  chondrocranium,  the 
statement  is  in  no  way  misleading. 

'These  dermal  plates  thus  form  an  almost  complete  case 
of  bone,  surrounding  and  protecting  the  internal  cartilaginous 
skull,  and,  by  supplying  the  deficiencies  of  the  latter,  effect  the 
complete  enclosure  of  the  brain  within  the  skeletal  parts. 
There  are  thus  formed  two  skulls,  one  within  the  other,  and  in 
the  ganoids,  where  the  relation  between  the  two  is  not  as  yet 
a  very  intimate  one,  the  outer  or  bony  skull  may  be  easily 
removed  from  the  other. 

The  next  step  in  advance  is  one  shown  also  among  the 
ganoids,  and  consists  of  the  strengthening  of  the  chondrocra- 
nium directly  by  the  development  of  centers  of  ossification 
within  the  cartilage  itself  (endochondral  ossification)  forming 
definite  osseous  elements,  called  from  their  mode  of  origin 
cartilage  bones,  in  distinction  from  the  other,  the  dermal 
bones.  Among  these  centers  may  be  enumerated  the  exoc- 
cipitals,  the  pro-oticsf  epiotics,  and  opisthotics,  which  together 
form  the  petrosals,  the  all-sphenoids,  the  orbito-sphenoids,  and 
the  ethmoids,  well-known  elements  in  the  skulls  of  higher 
vertebrates,  but  here  found  at  their  inception,  arising  as  iso- 
lated areas  of  the  chondrocranium  and  developing  at  the  ex- 
pense of  the  cartilage,  clearly  differing  from  the  dermal  bones 
in  origin. 

We  thus  find  in  the  skull  of  the  ganoids  the  elements  of 
the  vertebrate  skull  almost  at  their  beginning,  and  can  trace 
the  origin  of  parts  familiar  to  us  as  they  appear  in  the  spe- 
cialized skulls  of  mammals,  where,  under  the  cloak  of  an 
exactly  similar  external  appearance,  their  diverse  origin  has 
become  lost.  The  ganoids  seem  thus  a  vital  link  in  the  story 
of  the  skull,  yet  even  had  they  become  entirely  extinct,  as  they 


THE   ENDOSKELETON 


149 


came  very  near  being,  this  portion  of  the  history  might  have 
been  deciphered  from  the  embryological  records,  since  even  in 
the  mammals  the  primordial  skull  develops  from  its  primitive 
elements,  the  cartilage  bones  appear  as  centers  of  ossification 
within  it,  and  the  dermal  bones,  never  preformed  in  cartilage, 
appear  as  subcutaneous  ossifications  in  the  connective  tissue. 


A 


FIG.  39.  Two  views  of  the  skull  of  Cryptobranchus  allegheniensis,  a 
primitive  salamander,  a  little  higher  than  Necturus. 

CA)    Dorsal.      (B)   Ventral. 

DERMAL  BONES:  pm,  premaxillary;  mx,  maxillary;  n,  nasal?  f,  frontal;  pr.  f, 
pre-frontal;  p,  parietal;  sq,  squamosal;  pt,  pterygoid;  vp,  vomero-palatine;  pb,  para- 
basal. 

CARTILAGE  BONES:  os,  orbitosphenoid;  q,  quadrate;  ex.  o,  exoccipital  op,  operculum, 

OTHER  PARTS:  col,  columella;  nas,  nasal  capsule;  ec,  eye  capsule;  ot,  otic  cap- 
sule. 

In  both  figures  the  dermal  bones  have  been  retained  on  the  right  side  of  the 
skull  and  removed  on  the  left. 

In  that  case,  however,  we  could  hardly  have  obtained  an  idea 
of  the  appearance  of  the  adult  ganoids',  since  embryology,  with 
its  distortion  of  the  facts  through  an  early  assumption  of  the 
proportions  of  the  perfected  animal,  is  an  unsafe  guide  upon 
which  to  base  more  than  very  general  conclusions.  Had  the 
ganoids  been  lost  we  would  have  believed  in  a  general  way 
in  the  former  existence  of  fish-like  forms  in  which  the  dermal 
bones  were  still  in  the  form  of  an  exoskeletal  armature,  but 
their  exact  appearance  and  relationship  would  have  given  rise 


150  HISTORY   OF   THE    HUMAN    BODY 

to  endless  controversy,  such  as  always  occurs  with  regard  to 
places  where  the  records  are  incomplete,  and  this  vital  period 
in  the  history  of  the  skull  would  have  lost  much  of  its  reality. 

As  to  the  necessity  which  caused  the  appearance  of  these 
endochondral  ossifications  in  the  primordial  skull,  there  has 
been  pointed  out  a  curious  relationship  between  them  and  the 
principal  cranial  nerves,  namely,  that  the  ossifications  de- 
velop about  their  places  of  exit  from  the  brain  cavity  as  though 
to  protect  them.  Thus  we  have  the  olfactory  nerve  surrounded 
by  the  ethmoid,  the  optic  nerve  perforating  the  orbito-sphe- 
noid,  and  similar  relations  existing  between  the  trigeminus 
and  the  alisphenoid,  the  facial  nerve  and  the  prootic,  and  the 
ninth  and  tenth  and  the  exoccipital.  These  are  certainly  the 
topographical  conditions,  but  whether  a  causal  relation  really 
exists  between  them  is  not  known. 

In  completing  the  history  of  the  skull,  it  remains  to  no- 
tice the  ^amphibian  stage,  best  exhibited  by  urodeles,  and  the 
amniote  stage.,  typically  represented  by  reptiles  and  mammals. 
In  the  first  of  these  the  dermal  bones  are  no  longer  external  at 
any  stage  of  their  development  and  have  become  definitely 
incorporated  with  the  skull  as  physiological  parts  of  the  in- 
ternal skeleton.  Aside  from  this  the  characteristically  piscine 
elements,  like  the  rostrals,  the  orbitals  and  those  associated 
with  the  operculum,  have  become  lost,  and  the  bones  assume 
more  the  number  and  relationships  of  the  higher  terrestrial 
forms  (Fig.  19,  B,  and  Fig.  39). 

In  the  Amniota  one  of  the  fundamental  changes  is  the  loss 
of  the  parabasal  as  the  main  element  of  the  cranial  floor,  and 
its  functional  replacement  by  a  series  of  median  cartilage 
bones,  the  basi-o capital,  basi-sphenoid  and  pr<z-sphenoid. 
The  parabasal  may  be  entirely  lost,  but  in  the  light  of  recent 
investigation  it  seems  probable  that  it  is  continued  as  the 
median  vomer,  which  is  thus  not  the  same  as  the  paired  vo- 
merSj  which  form  a  portion  of  the  roof  of  the  mouth  of  fishes 
and  amphibians,  and  which,  if  this  view  is  the  correct  one, 
probably  disappear  in  amniotes. 

Another  characteristic  is  the  secondary  fusion  of  elements, 


THE   ENDOSKELETON  151 

and  as  the  purpose  of  this  procedure  is  wholly  physiological, 
the  object  being  to  insure  local  strength  or  gain  muscular 
attachments,  the  process  does  not  respect  origin,  but  often- 
times involves  both  dermal  and  cartilage  bones,  and  may  even 
include,  as  well,  elements  of  the  visceral  skeleton.  The  re- 
sults are  thus  bone-complexes,  each  of  which,  in  the  adult,  is 
a  morphological  puzzle,  to  the  history  of  which  we  have  in 
this  stage  no  clew.  Among  mammals  these  consolidations  are 
very  extensive,  especially  among  the  primates,  but  even  this 
condition  is  eclipsed  by  that  in  birds,  where  the  fusion  reaches 
its  extreme  and  nearly  all  of  the  bones  of  the  cranium  proper 
are  fused  in  the  adult  into  a  single  piece,  so  that,  in  order  to 
properly  study  the  skull,  observation  must  be  made  upon  a 
newly-hatched  fledgeling  or  even  upon  an  advanced  embryo 
extracted  from  the  egg. 

In  the  skulls  of  both  amphibians  and  amniotes,  even  in  the 
most  completely  ossified  ones,  there  remain  certain  portions 
of  the  unossified  chondrocranium,  especially  about  the  nose 
and  internal  ears.  In  mammals  there  develop  from  this  the 
cartilages  of  the  external  nose  which,  although  often  highly 
specialized,  are  to  be  considered  in  respect  to  origin  the  most 
ancient  parts  of  the  skull. 

In  giving  the  account  of  the  earlier  stages  in  the  develop- 
ment of  the  chondrocranium,  nothing  was  said  concerning  the 
subsequent  history  of  the  cartilage  of  the  eye-ball,  which 
could  not  unite  in  the  formation  of  the  skull.  In  many  fish 
the  outer  coating  of  the  eye-ball  (sclera)  remains  cartilaginous^ 
and  occasionally  becomes  very  thick  and  heavy  (e.  g.,  sword- 
fish).  There  are  also  traces  of  cartilage  in  the  sclera  of  many 
salamanders.  Both  cartilage  and  bone  occur  in  the  sclera  of 
certain  birds,  notably  hawks  and  owls,  in  the  latter  of  which 
a  series  of  long,  palisade-like  ossicles  forms  an  elongated  tube, 
shaped  something  like  the  tubes  of  an  opera-glass.  Whether 
these  are  the  direct  descendants  of  the  primitive  capsule  seems 
very  doubtful,  and  it  is  more  probable  that  these  develop- 
ments have  been  called  out  de  novo  in  response  to  functional 
necessity. 


152  HISTORY   OF   THE    HUMAN    BODY 

The  visceral  skeleton  is  associated  with  the  anterior  portion 
of  the  alimentary  canal  and  the  parts  derived  from  it,  and 
seems  to  have  developed  primarily  along  the  sides  of  the 
pharynx  for  the  regulation  of  those  most  primitive  of  verte- 
brate respiratory  organs,  the  gill-slits.  Although  Amphloxus 
possesses  a  very  regular  and  quite  complicated  system  of 
skeletal  bars  to  support  its  eighty  or  ninety  pairs  of  gill-slits, 
these  cannot  as  yet  be  brought  into  definite  relationship  with 
the  visceral  skeleton  of  the  higher  vertebrates ;  the  same  may 
be  said  of  the  visceral  skeleton  of  the  cyclostomes,  which  is  in 
the  form  of  a  complicated  pharyngeal  basket,  bearing  little 
apparent  resemblance  either  to  the  skeletal  bars  of  Amphioxus 
or  to  the  succession  of  simple  arches  characteristic  of  the 
fishes.  It  is  in  the  selachians  that  we  first  meet  with  a  vis- 
ceral skeleton  to  which  that  of  the  higher. forms  can  be  cer- 
tainly referred,  and  it  is  here,  therefore,  that  the  morphologi- 
cal history  of  the  vertebrate  visceral  skeleton  must  start. 

It  consists  of  a  series  of  pairs  of  cartilages,  more  or  less 
modified  from  the  form  of  simple  rods,  and  alternating  with 
gill-slits  that  open  from  the  exterior  into  the  pharyngeal 
cavity.  In  most  selachians  there  are  seven  well-developed 
pairs  of  these,  besides  a  few  cartilages  that  may  represent 
rudiments  of  others,  but  as  in  two  very  primitive  genera 
there  are,  respectively,  eight  and  nine  regular  pairs,  besides  the 
rudiments,  eleven  or  twelve  original  pairs  can  be  accounted 
for,  thus  suggesting  a  former  condition  with  a  large  number 
of  gill-slits,  a  supposition  that  compares  well  with  the  testi- 
mony furnished  by  Amphioxus.  The  selachian  condition, 
showing  all  the  pieces  that  may  be  accredited  to  the  visceral 
skeleton,  together  with  the  chondrocranium,  is  given  in  Fig. 
40,  somewhat  diagrammatized  from  an  actual  preparation. 
It  may  be  safely  assumed  that  at  one  time  these  visceral  arches 
were  all  similar  to  one  another,  associated  with  similar  gill- 
slits  and  all  gill-bearing,  although  important  modifications 
have  now  taken  place  in  certain  ones  of  them.  These  modifica- 
tions are  of  the  highest  importance  in  this  history  and  may 
now  be  considered,  with  constant  reference  to  the  figure. 


THE   ENDOSKELETON 


153 


The  first,  or  mandibular  pair,  which  is  the  most  modified 
of  all,  becomes  folded  about  the  mouth  in  such  a  way  as  to 
make  a  serviceable  pair  of  jaws,  both  upper  and  lower,  with 


IU 


B 


FIG.  40.  Skull  and  bran- 
chial skeleton  of  Squalus  acan- 
thias,  the  dog-fish,  after  draw- 
ings by  students. 

(A)  Lateral  view.  [C.  E.  Hep- 
burn.]  (B)  Ventral  view.  [J.  L. 
Whitney.] 

PPQ,  palato-pterygo-quadrate; 
Mk.  Meckel's  cartilage;  K.  I.  the 
two  sets  of  labial  cartilages;  HM, 
hyomandibular ;  CH,  cerato-hyal; 
Sp,  spiracular  cartilage;  BB,  basi- 
branchial;  HB,  hypo-branchial ;  CB. 
cerato-branchial ;  EB,  epi-branchial; 
PB,  pharyngo-branchial.  The  sub- 
script numerals  designate  the  sepa- 
rate gill  arches,  the  Roman  numer- 
als the  visceral  arches. 


YII 


each  of  which  several  rows  of  pointed  placoid  scales  are  as- 
sociated to  serve  as  teeth.  In  short,  one  hardly  knows  whether 
to  describe  these  parts  as  gill-arches  covered  with  placoid 


154  HISTORY    OF   THE    HUMAN    BODY 

scales,  or  as  jaws  equipped  with  teeth,  since  their  origin  as 
the  first,  and  their  present  and  future  function  as  the  second, 
are  both  so  clearly  indicated. 

We  have  thus  revealed  in  these  organs  a  valuable  bit  of 
history,  since  we  can  here  observe  the  jaws  and  teeth  of  ver- 
tebrates almost  at  their  birth,  and  can  learn  the  source  from 
which  the  material  was  derived  and  how  it  was  first  trans- 
formed. It  cannot  be  said,  however,  that  the  jaws  as  used 
here  have  developed  directly  into  those  of  the  higher  verte- 
brates, since  in  the  intermediate  history  there  is  much  addition 
of  new  material  and  replacement  of  old,  with  the  one  object 
in  view  of  increased  physiological  .effectiveness. 

The  assumption  of  an  upper  and  lower  jaw  marks  an  epoch 
in  early  vertebrate  history,  since  these  organs,  once  acquired, 
replace  forever  the  circular  jawless  mouth,  and  hood-like  lip, 
characteristic  of  both  Amphioxus  and  cyclostomes.  So  radi- 
cal must  have  been  the  change  that  some  think  that  even  the 
mouth  opening  is  a  different  one,  that  the  one  associated 
with  the  new  jaws  was  once  merely  a  gill-slit  like  the 
rest,  through  which  some  ancestor  acquired  the  habit  of  ad- 
mitting food,  and  that  its  manifest  advantage  over  the  other 
mouth  in  its  convenient  skeletal  equipment,  caused  the  dis- 
appearance of  the  old  one  and  the  perfection  of  the  new; 
there  are,  however,  certain  indications  that  point  to  the  former 
possession  of  a  still  older  mouth,  the  pal&ostoma,  homologous 
with  that  of  the  tunicate,  and  with  this  both  the  hood-like 
mouth  of  the  cyclostomes  and  the  slit  form  of  other  verte- 
brates may  be  contrasted  as  the  neostoma,  or  secondary  mouth. 
This  last  expression  applies  simply  to  the  opening,  which  is 
probably  homologous  in  all  vertebrates,  but  becomes  com- 
pletely metamorphosed  by  the  addition  of  jaws.  [Cf.  Chapter 
X.,  sub  Hypophysis.] 

The  use  of  placoid  scales  as  teeth  is  also  a  new  idea,  and 
they  are  certainly  a  great  advance  over  the  horny  epidermic 
excrescences  that  arm  the  circular  lip  of  the  cyclostomes.  Pla- 
coid scales  are  composite  structures,  composed  of  dentine  cov- 
ered over  by  enamel,  and  they  are  so  perfectly  fitted  for  thu° 


THE   ENDOSKELETON  155 

purpose  that  they  have  had  no  rivals  for  the  office;  thus,  al- 
though the  changes  in  form  and  arrangement  have  been  num- 
berless, the  teeth  of  even  the  highest  vertebrates,  composed  of 
dentine  overlaid  by  enamel,  attest  their  origin  from  placoid 
scales.  Aside  from  the  testimony  of  comparative  anatomy, 
the  embryonic  history  of  the  teeth,  even  in  the  highest 
form,  is  a  direct  corroborative  testimony  to  this  mode  of 
origin. 

Associated  with  the  first  pair  of  visceral  arches,  the  primi- 


tive jaws,  are  the  cartilages  of  the  second  pair,  the 
also  emancipated  from  the  function  of  bearing  gills  and  modi- 
fied in  part  to  assist  in  the  action  of  the  jaws.  Like  the  first, 
these  arches  also  consist  of  two  pieces,  a  dorsal  and  a  ventral 
one.  The  first,  called  the  hyomandibular,  is  more  or  less  de- 
tached from  the  other  and  forms  an  intermediate  piece,  tech- 
nically called  a  suspensorium,  between  the  cranium  and  the 
true  jaws.  To  this  is  also  attached  the  ventral  piece,  or  hyoid 
proper. 

The  remaining  five  arches,  the  genuine  branchial  arches, 
are  all  much  alike,  and  are  gill-bearing,  associated  with  gill- 
slits.  Each  one  consists  of  four  pieces,  two  dorsal^and  two 
ventral,  the  two  sets  bent  at  an  angle  with  each  other.  Along 
the  mid-ventral  lines  the  pairs  are  united  and  held  to  one 
another  by  median  pieces,  the  basi-branchials,  of  which  there 
is  typically  one  for  each  pair,  but  in  living  selachians  the  full 
number  is  seldom  represented.  The  additional  gill  arches  in 
the  two  primitive  forms  have  been  referred  to  above  and  are 
represented  in  the  diagram,  Fig.  41,  A,  by  dotted  lines. 

Aside  from  these  definite  and  well-developed  visceral  arches, 
as  they  may  be  called  collectively,  there  are  the  rudiments 
mentioned  above,  functionally  of  little  importance,  and  held 
by  some  to  be  the  reduced  remnants  of  still  other  arches. 
Such  are  the  labial  cartilages,  lying  at  the  angle  of  the  mouth 
and  used  to  strengthen  the  integumental  folds  of  the  lips; 
another  of  these  is  the  spiracular  cartilage,  crescentic  in  shape 
and  surrounding  the  spiracitlum  or  blow-hole,  perhaps  a  modi- 
fied gill-slit,  anterior  to  the  others.  These,  if  admitted  to  the 


156  HISTORY   OF   THE    HUMAN    BODY 

series,  will  considerably  increase  the  number  of  original  vis- 
ceral arches  and  render  more  probable  their  descent  from  a 
form  like  Amphioxus. 

In  the  other  Orders  of  fishes  the  visceral  skeleton  becomes 
somewhat  modified,  but  the  seven  pairs  of  visceral  arches  are 
recognizable  in  all  cases.  The  most  marked  changes  are  those 
affecting  the  jaws,  and  are  primarily  due  to  the  extensive  de- 
velopment of  dermal  bones,  which  reinforce  the  cartilaginous 
bars,  as  they  do  in  the  case  of  the  chondro cranium.  The 
lower  jaw  becomes  almost  entirely  encased  by  them,  the 
principal  dermal  elements  being  the  dentary,  which  covers  the 
outside  and  bears  the  most  or  all  of  the  teeth,  and  the  angu- 
lare,  which  covers  the  inner  side. 

Lying  within  these,  as  if  bound  in  splints,  is  the  cartilagi- 
nous lower  jaw,  the  original  visceral  arch,  which  is  destined 
from  now  on  to  lose  its  functional  importance  save  at  its 
posterior  end,  which  here  emerges  from  the  splints  and  pre- 
sents a  rounded  articular  surface.  This  piece  is  called  the 
articular et  and  sometimes  ossifies,  forming  a  cartilage  bone. 
The  entire  cartilaginous  arch,  thus  subordinated,  the  mandibu- 
lar  cartilage,  is  also  called  Meckel's  cartilage,  the  name  com- 
memorating the  distinguished  German  anatomist,  Johann 
Friedrich  Meckel  [1781-1833],  who  first  saw  it  in  the  embryo 
human  jaw,  lying  encased  in  the  dermal  bones,  much  as  in  the 
adult  ganoid  or  teleost. 

The  original  upper  jaw,  however,  loses  still  more  prestige, 
since  its  function  as  a  jaw  is  entirely  usurped  by  a  set  of 
dermal  bones,  the  prcemaxillary  and  maxillary,  placed  paral- 
lel to,  but  outside  of  it.  In  spite  of  this,  however,  it  becomes 
directly  encased  by  other  dermal  bones,  the  palatine  and  the 
pterygoid,  and  is  retained  as  an  accessory  upper  jaw  in  some 
fishes  and  a  few  salamanders.  Its  posterior  end,  like  that  of 
the  lower  jaw,  remains  free  and,  after  the  reduction  of  the 
anterior  portion,  fits  in  between  the  hyomandibular  and  the 
articulare  of  the  lower  jaw  as  an  extra  suspensory  piece.  In 
later  development  it  ossifies  as  a  cartilage  bone,  under  the 
name  of  quadrate.  As  for  the  ultimate  fate  of  the  anterior 


THE   ENDOSKELETON  157 

portion,  the  dermal  palatine  and  pterygoid  enlarge  at  its  ex- 
pense, and  it  is  retained  as  an  unimportant  bit  of  cartilage 
in  some  amphibians,  but  beyond  these  it  is  not  seen.  The  two 
dermal  pieces,  which  originally  encased  it,  however,  retain  con- 
siderable importance  and  usually  appear  in  the  skulls  of  am- 
niotes  along  a  curve  approximately  parallel  to  that  of  the 
maxillaries,  but  interior  to  it,  thus  marking  the  former  po- 
sition of  the  lost  cartilage.  These  bones  are  often  tooth- 
bearing  in  fishes  and  amphibians,  retaining  this  much  of  their 
former  function. 

Of  the  five  pairs  of  true  branchial  arches,  the  first  four 
are  retained  in  ganoids  and  teleosts  as  gilf-bearers,  while  the 
fifth  lies  in  the  floor  of  the  pharynx  and  is  often  covered 
with  sharp  teeth  arranged  in  several  rows. 

A  notable  modification,  which,  however,  is  mainly  an  ex- 
ternal one,  is  found  in  the  development  of  the  opercuhim  or 
gill-flap,  which  appears  as  a  fold  of  skin  and  becomes  rein- 
forced by  special  dermal  bone.  This  ultimately  develops  pos- 
teriorly so  as  to  cover  all  the  gill-slits  in  such  a  way  that  they 
seem  to  be  internal,  and  are  not  visible  from  the  exterior,  as 
in  the  case  of  most  selachians. 

The  subsequent  history  of  the  visceral  skeleton  and  its  fate 
in  amphibians,  reptiles,  birds,  and  mammals  may  be  quickly 
outlined.  These  Classes  are  fundamentally  terrestrial,  and 
never  possess  genuine  internal  gills,  and  thus  the  main  changes 
are  due  to  a  loss  of  function,  which,  by  throwing  the  branchial 
arches  out  of  employment,  would  have  caused  them  to  disap- 
pear, were  it  not  that  they  could  be  in  part  employed  to  sub- 
serve some  other  necessary  function.  It  will  be  seen  that  in 
most  cases  this  has  been  their  fate,  but  this  very  change  of 
function,  although  it  saves  them  from  complete  destruction, 
of  necessity  causes  considerable  modification,  oftentimes  a  pro- 
found one. 

Beginning  with  the  most  anterior  arches,  the  mandibular 
and  hyoid,  the  hyomandibular  seems  to  disappear  entirely,  al- 
though doubtfully  identified  by  some  morphologists  with  cer- 
tain other  elements  in  the  otic  region,  and  leaves  to  the  quad- 


158 


HISTORY   OF   THE    HUMAN    BODY 


FIG.  41.  Morphology  of  the  visceral  skeleton. 

(A)  Selachian.  (B)  Amphibian.  (C)  Sauropsidan.  (D)  Mammal. 
In  all  the  figures  the  visceral  arches  are  designated  by  Roman  numerals;  in  the 
case  of  the  first  two  the  dorsal  and  ventral  segments  are  further  designated  by  ex- 
ponent letters  (d  and  v).  Other  designations  are,  Ib,  labial  cartilage;  s,  spiracular 
cartilage;  o,  operculum.  In  arch  VII  the  .  arytaenoid  and  tracheal  cartilages  are 
designated  by  the  exponents  a  and  b,  respectively.  The  designations  for  the  stapes 
represent  an  interpretation  at  variance  with  the  one  given  in  the  text,  but  formerly 
widely  accepted. 


THE   ENDOSKELETON  159 

rate  the  responsible  role  of  being  the  only  suspensory  piece 
for  the  mandible.  As  a  cartilage  bone  it  persists  in  am- 
phibians, reptiles,  and  birds.  The  several  elements  of  the 
mandible  remain  distinct  in  amphibians  and  reptiles,  but  con- 
solidate in  birds,  the  proximal  end,  which  articulates  with  'the 
quadrate,  being  in  all  cases  the  free  posterior  end  of  Meckel's 
cartilage  (=articulare).  In  mammals  a  great  change  takes 
place  in  these  parts,  the  history  of  which  is  repeated  in  the  de- 
veloping embryo,  through  which  the  facts  first  came  to  light. 
Here  both  quadrate  and  articulare,  external  at  first,  as  in  am- 
phibians and  reptiles,  become  drawn  into  the  tympanic  cavity 
(middle  ear) where,  still  retaining  approximately  their  original 
shape,  though  proportionately  reduced  in  size,  they  become  the 
incus  and  malleus,  respectively,  while  the  mandible,  each  half 
consolidated  into  a  single  bone,  forms  a  new  articulation  di- 
rectly with  the  skull  in  the  petrosal  region.  The  old  articula- 
tion of  the  mandible,  that  between  quadrate  and  articulare,  now 
incus  and  malleus,  after  having  served  so  long  and  well  in  the 
mastication  of  food,  emancipated  from  this-  coarse  work  and 
remaining  almost  embryonic  in  point  of  size,  becomes  attuned 
to  sound  waves  and  assists  in  their  transmission!  The  third 
and  innermost  bone  of  the  tympanic  cavity,  the  stapes,  has 
been  for  a  long  time  a  true  bone  of  contention,  in  spite  of  its 
small  size.  Some  authorities  have  attempted  to  identify  it 
with  the  missing  hyomandibular,  the  dorsal  half  of  the  second 
visceral  arch,  which  disappears  above  the  fish.  It  appears, 
however,  to  have  had  a  double  origin,  one  for  the  loop,  the 
other  for  the  base.  The  first  seems  to  have  been  originally  de- 
rived in  amphibians  from  the  cartilaginous  wall  of  the  otic 
capsule,  and  to  be  thus  a  part  of  the  chondrocranium,  and  not 
an  element  of  the  visceral  system.  The  oval  base  is  an  ossified 
membrane,  secondarily  fused  with  the  other  piece.  The  fora- 
men in  this  minute  bone,  to  which  it  owes  its  stirrup-like  shape 
in  man  and  in  some  other  mammals,  transmits  an  artery  which 
in  man  disappears  in  the  embryo,  but  in  Insectivora  and  rodents 
is  retained  throughout  life,  the  arteria  stapedialis. 

The  remaining  arches  subserve  in  part  the  original  function 
of  gill-bearers  so  long  as  there  is  opportunity,  which  occurs 


160  HISTORY    OF   THE    HUMAN    BODY 

only  in  a  few  amphibians,  and  then  mainly  during  larval  life, 
and  are  otherwise  modified  to  assist  in  the  functions  performed 
by  two  organs  that  develop  in  the  region,  the  tongue"  and  the 
larynx.  The  latter  makes  its  appearance  in  a  very  simple 
condition,  associated  with  two  bag-like  lungs,  in  the  most 
primitive  of  the  salamanders,  and  utilizes  as  the  first  laryngeal 
cartilage,  cartilago  lateralis,  the  last  of  the  gill-arches  (5th 
branchial  or  7th  visceral  arch).  This  element,  which,  by  sub\ 
division  and  metamorphosis,  develops  into  a  pair  of  arytcenoid 
cartilages  and  a  series  of  lateral  trachea!  pieces,  shows  itself 
capable  of  becoming  a  complicated  mechanism,  sufficient  for 
the  needs  of  amphibians;  but  in  the  reptiles  the  4th  gill-arch 
(6th  visceral)  becomes  associated  with  it  as  the  epiglottis, 
which  here  appears  for  the  first  time.  The  reptilian  larynx, 
with  but  little  modification,  is  employed  by  the  birds,  but  in 
mammals  the  next  two  gill-arches,  counting  anteriorly,  the 
2nd  and  3rd  (4th  and  5th  visceral)  become  associated  together 
in  front  of  the  larynx  and  form  the  protecting  shield-like 
piece,  the  thyreoid,  which  in  the  lowest  Order  (Monotremata) 
still  appears  like  two  pairs  of  arches  covering  the  larynx 
ventrally. 

It  may  be  said  in  general  that  in  all  the  Classes,  from 
the  amphibians  on,  those  visceral  arches  not  employed  as 
jaws  or  as  laryngeal  cartilages  form  a  hyoid  or  hyo-branchial 
complex  and  furnish  a  skeletal  equipment  for  the  tongue,  an 
organ  which  often  develops  voluminously,  fitted  for  very  spe- 
cial work.  Thus,  in  the  amphibians  in  general  this  complex 
consists  of  the  2nd  to  the  6th  visceral  arches,  inclusive;  in 
reptiles  and  birds  of  the  2nd  to  the  5th,  and  in  mammals  of 
the  2nd  and  3rd  only.  In  the  latter  these  two  remaining 
arches  form  a  complex  consisting  of  a  median  piece, 
the  basi-hyal,  and  two  pairs  of  cornua,  the  anterior  pair 
representing  the  2nd  visceral  arch,  the  true  hyoid  of  fishes 
and  the  posterior  pair  the  3rd  visceral  or  ist  gill-arch.  In 
most  mammals  the  anterior  cornua  of  this  hyoid  complex  con- 
sist of  a  chain  of  small  bones,  which,  enumerated  from  the 
basi-hyal  ("body  of  the  hyoid"),  are:  cerato-hyal,  epi-hyalf 


THE   ENDOSKELETON  161 

stylo-hyal,  and  tympano-hyal,  the  last  closely  associated  with 
the  external  opening  of  the  ear.  In  man  and  the  higher  apes 
the  two  latter  are  fused  with  the  skull  to  form  the  ff  styloid- 
process  of  the  temporal  bone,"  and  this  is  connected  with 
the  cerato-hyal  (="  lesser  or  anterior  cornua  ")  by  the  stylo- 
hyoid  ligament ,  which  replaces  the  missing  epi-hyal.  Oc- 
casionally a  rudiment  of  this  latter  bone  is  found  in  the  middle 
of  the  ligament. 

A  recently  described  and  very  singular  metamorphosis  of 
a  portion  of  a  visceral  arch  is  that  by  which  in  mammals  the 
outer  end  of  the  2nd  or  hyoid  arch,  naturally  located  near  the 
external  opening  of  the  ear,  segments  off  and  becomes  trans- 
formed into  the  cartilage  of  the  external  ear-flap,  the  auricula 
or  pinna,  which  in  the  various  Orders  responds  readily  to  the 
varied  environments  of  different  mammals  and  exhibits  a 
great  range  of  variation  in  shape  and  size. 

Of  what  a  series  of  changes  and  unexpected  metamorpho- 
ses has  the  visceral  skeleton  shown  itself  capable,  and  what 
vicissitudes  have  the  several  elements  experienced !  Beginning 
as  a  set  of  similar  arches,  regulating  the  opening  and  closing 
of  gill-slits,  they  become  jaws,  vocal  organs,  supports  for  the 
tongue,  suspensory  pieces  for  the  mandible,  tympanic  ossicles 
and  flapping  external  ears.  Indeed,  a  well-recognized,  though 
not  generally  accepted,  theory  derives  from  them  also  the 
skeleton  of  the  free  limbs,  including  the  shoulder  and  hip 
girdles,  and  even  the  long  bones  of  the  limbs  and  the  numer- 
ous smaller  pieces  in  carpus  and  tarsus  and  in  the  digits.  But 
even  without  this  latter  theory,  which  appears  untenable,  the 
subject  presents  a  remarkable  history  of  repeated  change 
of  function,  of  the  adaptation  of  old  material  to  new 
purposes,  of  the  dethronement  of  the  old  systems  and 
the  employment  of  their  organs  for  the  development  of 
the  new;  we  see  here  Nature  constantly  exploring  new 
environments,  and  then  adapting  function  to  environment,  and 
material  to  function,  constantly  making  over  old  organs  in 
obedience  to  mechanical  laws  and  never  originating  or  creating 
new  ones  de  novo.  The  results  are  thus  often  imperfect  and 


162  HISTORY   OF   THE   HUMAN    BODY 

incomplete,  and  are  frequently  obtained  by  a  very  indirect 
and  circuitous  route,  and  although  many  an  animal  form  and 
many  myriads  of  individuals  have  succumbed  as  the  result 
of  some  mechanical  disadvantage  which  the  skill  of  a  simple 
human  mechanic  could  have  remedied,  there  is  never  the  least 
sign  or  indication  of  such  an  interference.  Everything  de- 
velops as  an  inevitable  result  of  natural  law,  as  a  part  of 
the  general  plan  which  is  broad  enough  to  include  the  entire 
universe  and  which  is  willing  to  sacrifice  countless  hecatombs 
of  lives  rather  than  submit  to  a  single  exception  to  its 
laws. 

In  about  the  same  degree  as  the  visceral  skeleton  of  the  true 
vertebrates  is  suggested  by  the  gill  armature  of  Amphioxus, 
so  do  its  fin-folds  and  the  rows  of  spines  enclosed  by  them 
suggest  a  simple  condition  from  which  may  be  derived  the 
appendicular  or  limb  skeleton.  In  Amphioxus  a  continuous 
though  very  low  fin-fold  begins  near  the  anterior  end,  runs 
the  entire  length  of  the  animal,  and  is  continuous  around  the 
tail  with  a  median  ventral  one  as  far  forward  as  the  atriopore, 
where  it  divides  into  two,  which,  as  the  metapleural  folds, 
continue  almost  to  the  mouth. 

This  fold  system  is  supported  throughout  its  entire  length 
by  a  skeleton  of  gelatinous  fin-spines,  thus  forming  an  ap- 
paratus, which,  if  developed  slightly  more  than  in  Amphioxus, 
would  be  very  serviceable  in  retaining  the  equilibrium  while 
swimming,  serving  as  dorsal  and  ventral  keels.  The  skeletal 
fin-rays  would  become  developed  as  well  as  the  external  folds, 
and  the  general  appearance  would  be  not  unlike  that  sug- 
gested by  Fig.  42,  a.  It  will  be  noticed  that  in  Amphioxus 
certain  parts  of  the  caudal  fin  are  wider  than  the  rest,  showing 
how  responsive  this  fold  is  to  a  localized  increase  of  function, 
and  this  allows  one  to  draw  the  hypothetical  ancestor  with 
certain  areas  of  the  fin-fold  marked  by  a  greater  width,  corre- 
sponding to  the  places  where  the  greatest  stress  would  be  apt 
to  come  in  an  actively  swimming  animal.  As  a  farther  de- 
parture from  Amphioxus,  the  division  of  the  median  ventral 
fin  takes  place  at  the  anus,  and  not  at  an  atriopore,  since  this 
latter  is  probably  a  special  organ  developed  to  supply  the 


THE   ENDOSKELETON 


163 


needs  of  Amphioxus,  and  not  transmitted  to  any  of  its  de- 
scendants. 

Judging  from  the  final  results  as  we  see  them  in  the  great 
Class  of  fishes,  it  may  be  supposed  that  the  inequalities  in  the 
development  of  this  fin-fold  increased  through  localised  func- 


FIG.  42.  Diagrams  illustrating  the  fin-fold  theory. 

(a)  and  (b)  [after  WIEDERSHEIM]  represent  the  unmodified. theory.  A  continuous 
fin-fold,  stiffened  by  skeletal  rays,  extends  along  the  median  dorsal  line,  around  the 
tail,  and  along  the  mid-ventral  line  as  far  as  the  cloaca,  where  it  divides  into  two 
lateral  folds  that  extend  along  the  sides  of  the  trunk.  The  retention  of  portions 
of  this  fold  and  the  loss  of  the  intermediate  portions  results  in  the  formation  of  both 
median  and  paired  fins.  In  (c)  [after  RABL]  is  shown  RABL'S  modification  of  this 
theory.  The  two  lateral  folds  are  from  the  beginning  distinct  from  the  median  one, 
and  are  hence  subjected  to  external  influences,  especially  at  their  free  anterior  and 
posterior  ends,  thus  modifying  the  first  and  last  rays  the  most,  and  the  others  in 
a  progressively  decreasing  series  towards  the  middle  of  the  fold.  When,  later,  the 
first  and  the  last  portions  become  set  off  (along  the  dotted  lines)  and  the  interme- 
diate portion  suppressed,  they  form  fins,  of  which  the  anterior  one  shows  greater 
modifications  along  its  anterior,  the  other  along  its  posterior  border,  precisely  as 
is  the  actual  case  among  fishes. 

tional  activity,  until  certain  portions  became  especially  ^vell 
developed,  while  the  intermediate  portions  were  entirely  lost. 
(Fig.  42,  b).  The  significance  of  this  is  seen  if  this  figure  be 
now  compared  with  any  good,  typical  fish,  which  shows  the 
perfected  type  resulting  from  this  process.  Here  the  fins  are 
all  alike  in  structure,  proving  their  derivation  from  a  common 
origin,  but  are  divided  into  two  groups  in  accordance  with 
their  position,  median  and  paired.  Of  the  median  fins  the 


164  HISTORY   OF    THE    HUMAN    BODY 

dorsal  (one  or  more)  and  the  anal  function  as  keels  to  re- 
tain the  equilibrium  and  prevent  the  body  from  rolling  side- 
ways, and  the  caudal  fin  is  the  main  organ  of  propulsion,  but 
may  act  in  part  also  as  a  rudder  to  regulate  the  direction  in 
which  the  animal  moves.  The  paired  fins,  which  bear  the  in- 
appropriate names  of  pectorals  and  ventrals,  act  as  subsidiary 
oars  or  paddles  and  seem  mainly  to  guide  and  maintain  the 
course. 

A  recent  suggestion,  which  serves  as  an  addition  to  the  fin- 
fold  theory,  is  given  in  Fig.  42,  c,.in  which  it  is  supposed  that 
the  lateral  folds  are  primarily  distinct  from  the  median  one, 
and  that  the  paired  fins  develop  from  their  free  extremities, 
where  the  stress  of  motion  naturally  comes.  This  furnishes 
a  reason  other  than  chance  for  the  formation  of  two,  and  only 
two,  pairs  of  fins,  and  also  explains  a  sort  of  symmetry  shown 
in  the  two  sets  of  fins  of  many  fishes,  since  the  free  edge, 
and  hence,  the  strongest  development,  is  anterior  in  the  for- 
ward fin  and  posterior  in  the  back  one. 

The  median  fins,  being  of  use  only  in  the  water,  disappear 
above  the  fish,  although  the  necessity  of  similar  organs  for 
aquatic  life  is  well  shown  by  the  fact  that  in  secondarily  aquatic 
higher  vertebrates  which  have  returned  to  the  water  although 
derived  from  a  terrestrial  ancestry,  some  new  form  of  median 
fins  becomes  developed.  Such  animals  have  lost  the  serviceable 
median  fins  of  fishes,  with  their  strong  skeletal  spines,  and,  as 
they  cannot  recall  them,  are  forced  to  develop  some  make- 
shift arrangement  to  serve  the  purpose.  Thus,  aquatic  am- 
phibians develop  a  tail  fin  of  integument  without  skeletal  sup- 
port, the  tail  of  the  crocodile  is  supplied  with  keels  formed  of 
projecting  scales,  and  the  whale  has  manufactured  perfect 
dorsal  and  caudal  fins  out  of  whole  cloth,  as  it  were,  since  they 
are  made  from  thick,  though  hairless,  mammalian  integument, 
reinforced  by  connective  tissue.  The  dorsal  fin  of  this  latter, 
as  is  the  case  with  other  external  details,  is  wonderfully  fish- 
like,  but  the  caudal  fin  is  flattened  the  other  way,  and  extends 
laterally,  instead  of  up  and  down,  as  in  fishes. 

To  summarize,  then,  the  original  median  fins  of  fishes  have 


THE   ENDOSKELETON  165 

developed  to  subserve  the  needs  of  an  aquatic  life,  and  disap- 
pear forever  with  the  assumption  of  a  terrestrial  environment, 
although,  in  secondarily  aquatic  forms,  similar  organs  are  de- 
veloped from  other  sources.  The  paired  iins,  on  the  other  hand, 
useful  in  fishes,  assume  their  highest  importance  on  land,  and 
become  the  two  pairs  of  free  limbs.  To  these  more  than  to 
any  other  organ,  the  higher  vertebrates  owe  their  extraordi- 
nary development,  and  their  high  degree  of  success  in  occu- 
pying so  many  sorts  of  environment,  since  by  this  means  they 
have  been  able  to  possess  the  surface  of  the  earth,  to  occupy 
the  trees,  to  burrow  in  the  ground,  to  return  to  the  sea  and 
even  to  conquer  the  problem  of  aerial  navigation.  It  is,  more- 
over, probable  that  the  emancipation  of  the  fore  limbs  from 
the  function  of  locomotion  and  the  acquirement  by  them  of 
prehensible  powers,  which  enable  them  to  seize  objects  and 
bring  them  to  the  immediate  attention  of  the  sense  organs, 
have  been  the  chief  causes  of  the  excessive  brain  development 
which  has  achieved  for  the  Primates  the  greatest  success  thus 
far  attained  in  the  domination  of  the  world. 

Corresponding  to  the  great  variety  of  functions  of  which 
the  paired  limbs  are  capable,  there  is  an  equally  vast  series  of 
modifications  of  structure,  presenting  an  army  of  forms  which 
include  various  sorts  of  ambulatory  limbs,  paddles,  grasping 
organs,  tools  for  excavating  the  earth,  and  wings  of  several 
sorts.  Among  the  modifications  there  must  be  included  also 
the  numerous  cases  of  limb  reduction,  which  may  affect  the 
fore  or  hind  limbs  or  both,  and  exhibit  every  stage  of  reduc- 
tion down  to  total  loss.  Thus  the  amphibian  Siren  has  lost 
its  hind  legs  totally,  while  retaining  its  fore  legs,  and  simi- 
larly in  the  case  of  the  whale  the  fore  legs  become  developed 
into  enormous  flippers  or  paddles,  while  the  hind  legs  are  re- 
duced to  useless  rudiments  entirely  concealed  beneath  the 
skin.  The  opposite  tendency  is  seen  in  the  ostrich,  where  the 
legs  are  very  short  and  heavy,  while  the  wings  are  much  re- 
duced and  almost  without  function,  and  is  still  better  exhibited 
in  the  kiwi-kiwi  of  New  Zealand,  in  which  the  wings  have 
totally  disappeared.  In  several  groups  the  complete  or  nearly 


166  HISTORY   OF   THE   HUMAN    BODY 

.complete  reduction  of  both  pairs  is  correlated  with  an  exces- 
sive lengthening  of  the  body,  locomotion  being  effected  by 
.an  undulatory  movement  of  the  entire  animal.  In  snakes, 
.which  progress  mainly  through  the  action  of  their  very 
numerous  ribs,  the  loss  of  both  pairs  of  limbs  is  usually 
a  total  one,  but  in  the  boas  the  posterior  limbs  ap- 
pear as  spur-like  rudiments,  situated  upon  either  side  of  the 
cloacal  orifice,  and  are  of  considerable  use  in  climbing  trees. 
Aside  from  the  snakes  a  similar  form  is  assumed  by  several 
groups  of  reptiles  and  amphibians,  the  adaptation  fitting  them 
in  some  cases  for  a  life  similar  to  that  of  snakes,  and  in  others 
for  a  subterranean  existence.  These  latter,  which  include  at 
least  one  group  of  lizards  and  one  of  amphibians,  burrow  in 
the  earth  like  earth-worms,  and  as  in  the  process  of  this 
adaptation  they  have  lost  their  eyes,  reduced  their  head  and 
arranged  their  scales  in  the  form  of  annular  segments,  they 
resemble  these  latter  animals  almost  to  the  point  of  deception. 
Through  all  their  vicissitudes,  however,  the  number  of  free 
limbs  is  constant  in  all  vertebrates,  except  when  secondarily 
reduced,  and  consists  of  two  pairs,  corresponding,  as  we  sup- 
pose, to  the  number  of  original  points  at  which  the  primitive 
fin-fold  became  hypertrophied.  Although  this  number  may 
be  reduced  as  a  special  adaptation,  it  can  never  be  increased, 
and  the  favorite  mythological  conceptions  of  human  and  other 
vertebrate  forms  with  supernumerary  limbs  are  far  more  im- 
possible and  absurd  than  is  usually  recognized,  since  they  are 
generally  held  to  be  merely  contrary  to  experience,  but  are 
here  seen  to  violate  the  fundamental  principles  of  develop- 
ment.* It  might,  indeed,  have  been  possible  in  the  first  place 
for  the  lateral  fin-folds  to  have  hypertrophied  in  three  or  more 
places  instead  of  two,  a  result  which  some  slight  change  of 
environment  or  habit  would  have  then  easily  effected,  but  the 

*  Cases  of  monstrosities  with  extra  limbs,  in  which  the  total  number  ex- 
ceeds four,  are  not  violations  of  this  principle,  but  are  anomalies  due  to  a 
multiplication  of  certain  of  the  anlagen,  like  monsters  with  two  heads  or 
two  bodies.  The  cause  of  such  redundancy  is  as  yet  imperfectly  under- 
stood, but  enough  has  been  already  proven  to  show  that  it  lies  in  the 
germ,  in  which  the  abnormality  probably  exists  in  the  form  of  redundant 
germinal  elements. 


THE    ENDOSKELETON  167 

time  for  accomplishing  this  is  now  long  past  and,  the  number^ 
of  limb-anlagen  once  established,  no  subsequent  change  is  pos-f 
sible.  Here  again  the  principle  is  clearly  enunciated  that 
there  is  never  any  anticipation  for  the  future  in  the  history 
of  development,  and  that  each  step  is  taken  with  sole  refer- 
ence to  the  needs  of  the  animal  that  takes  it.  Although  a 
given  form  is  destined  to  be  the  ancestor  of  a  great  group  of 
higher  animals,  yet  there  is  no  prescience  exhibited  in  the 
details  of  its  structure  other  than  the  needs  of  its  own  economy, 
and  the  task  of  laying  down  the  lines  in  accordance  with  which 
its  numberless  descendants  are  to  be  constructed  is  left  to 
the  chance  of  the  necessary  adaptations.  These  conditioning 
characteristics  may  or  may  not  be  the  best  for  the  future,  but 
in  either  case  they  are  transmitted  to  posterity,  to  grant  them 
success  or  failure,  as  the  case  may  be. 

The  form  assumed  by  the  free  paired  limbs  throughout  the 
Class  of  fishes  is  that  of  a  fin  or  ichthyopterygium,  a  type  con- 
sisting of  a  thin  double  membrane  supported  by  a  variable 
number  of  fin-rays;  a  single  type  also  underlies  the  countless 
modifications  exhibited  by  the  higher  vertebrates,  the  hand- 
form  or  chiropterygium,  a  type  consisting  of  three  main  di- 
visions, proximal,  medial,  and  distal,  the  last  terminating  in 
five  digits.  Simple  as  it  is  to  refer  all  modifications  existing 
in  higher  vertebrates  to  the  latter,  and  in  the  fishes  to  the 
former  type,  no  satisfactory  explanation  has  thus  far  been 
forthcoming  to  bridge  the  wide  gap  existing  between  the  two. 
That  the  two  possess  an  independent  origin  would  involve  the 
total  suppression  of  the  appendicular  apparatus  formed  from 
the  fin-fold  and  the  development  de  novo  of  two  pairs  of  ap- 
pendages in  the  same  place,  and  for  the  same  or  a  similar 
purpose,  a  supposition  which  involves  too  much  improbability 
to  be  considered  for  a  moment.  The  free  limbs  of  the  one  type 
must  be  strictly  homologous  with  those  of  the  other,  and  the 
fact  of  the  present  distinctness  of  the  two  types  is  undoubtedly 
due  to  the  extinction  of  transition  forms.  This  transition  must 
have  taken  place  at  the  epoch  at  which  the  vertebrates  first 
attained  the  land,  a  transition  which  must  have  been  a  com- 


i68 


HISTORY   OF   THE    HUMAN    BODY 


paratively  abrupt  one,  characterized  by  a  rapid  adjustment  to 
the  needs  of  a  terrestrial  life.  It  thus  follows  that  the  transition 
forms  themselves  must  have  been  put  at  a  disadvantage  when 
in  competition  both  with  their  immediate  descendants,  which 
were  better  fitted  for  the  land,  and  with  their  immediate  ances- 
tors, which  had  never  left  the  water,  and  their  rapid  extinction 
was  a  necessary  consequence.  The  deficiency  in  the  historical 
record  at  this  place,  however,  has  not  prevented  speculation  on 
this  subject;  on  the  contrary,  it  has  proved  an  especially  at- 


FIG.  43.  Diagrams  illustrating  the  development  of  the  fin  skeleton; 
based  on  that  of  selachians.  [After  WIEDERSHEIM.]  In  (a)  and  (c) 
the  right  side  shows  a  slightly  older  stage  than  the  left. 

tractive  field  for  the  anatomical  philosopher,  and  the  discus- 
sion of  some  of  the  leading  theories  of  this  question  will  be 
considered  farther  on  in  the  present  chapter. 

Aside  from  this  problem,  however,  the  history  of  the  de- 
velopment of  the  limbs  is  by  no  means  clear  in  other  respects, 
and  although  the  faith  in  their  complete  homology  throughout 
is  universal,  the  manner  of  their  development  and  the  relation 
of  the  various  forms  to  one  another  cannot  be  agreed  upon. 
Embryology,  which  is  usually  so  suggestive,  is  practically 
silent  here,  since  the  record  seems  in  all  cases  to  be  much 
abbreviated.  The  best  that  can  be  done,  therefore,  is  to 
arrange  a  sequence  of  adult  forms  which  seem  to  show 
transitions  from  one  type  to  another,  paying  as  much  regard 
as  possible  to  the  lines  of  descent  as  indicated  by  the  other 
parts.  In  this  way  have  been  sketched  the  histories  which 
follow,  and  in  reading  this  it  must  be  remembered  that  a  his- 
tory founded  merely  on  a  succession  of  adult  forms,  and  re- 


THE   ENDOSKELETON  169 

maining  unsubstantiated  by  a  parallel  series  of  embryonic 
stages,  rests  upon  an  insecure  foundation  and  is  liable  to  re- 
ceive considerable  modification  through  the  discovery  of  ad- 
ditional facts.  In  tracing  these  histories  the  anterior  and 
posterior  limbs  must  be  treated  separately,  since,  although  the 
free  limbs  are  plainly  serially  homologous,  and  often  corre- 
spond quite  closely,  part  for  part,  the  girdles,  although  equally 
a  portion  of  the  appendicular  skeleton,  differ  fundamentally 
from  one  another  and  must  have  had  a  somewhat  different 
early  history. 

Beginning  with  the  posterior  limb,  which  is  more  conserva- 
tive than  the  anterior,  and  probably  retains  more  primitive 
characteristics,  it  may  safely  be  supposed  that  at  its  origin  as 
a  localized  flap  ofj^once  continuous  fin-fold,  its  skeleton  con- 
sisted of  a  series  of  spines  or  rays,  independent  of  one  another, 
and  somewhat  longer  at  their  bases,  tapering  to  their  free 
extremities  (Fig.  43,  a).  To  insure  strength  and  to  gain  a 
concerted  action,  a  very  natural  step  would-be  to  widen  these 
at  their  bases  still  farther  until  they  fuse,  forming  a  piece 
something  like  a  comb,  with  long  teeth  far  apart  (Fig;  43,  b). 
As  these  organs  become  of  still  greater  importance  and  need 
a  firmer  support,  the  basal  portions,  corresponding  to  the 
backs  of  the  combs,  would  be  likely  to  grow  inwards  until 
they  meet  and  fuse  across  the  mid-ventraMine^  thus  forming 
a  very  primitive  girdleTwitrT which  the  free  part  would  become 
movably  articulated  (Fig.  43,  c).  This  last  case,  is,  however, 
not  a  hypothetical  one,  but  drawn  directly  from  the  posterior 
girdle  and  free  limb  skeleton  of  the  dog-fish,  a  typical  se- 
lachian, in  which  the  free  limb  consists  of  a  basipterygium, 
bearing  a  series  of  rays,  the  whole  being  movably  attached  to  a 
girdle  in  the  form  of  a  ventral  band.  Although  there  is  at 
present  an  impassable  space  between  this  free  limb  and  that 
of  even  the  simplest  amphibian,  the  girdles  of  the  two  forms 
are  not  so  far  apart,  since  a  broadening  of  the  middle  piece 
into  a  plate,  and  the  extension  of  its  ends  dorsally  until  they 
come  in  contact  with  a  pair  of  ribs,  would  convert  the  one 
into  the  other.  (Cf.  Fig.  44,  d.) 


170 


HISTORY   OF   THE   HUMAN    BODY 


The  fault  in  the  above  theory  is  that,  while  offering-  a  direct 
transition  from  the  selachian  to  the  amphibian  form,  it  leaves 


FIG.  44.  Series  illustrating  a  theory  of  the  phylogenetic  development 
of  the  pelvic  girdle.  [Mainly  after  WIEDERSHEIM.] 

(a)  Acipenser  (sturgeon),  (b)  Scaphyrhynchus  (shovel-nosed  ganoid),  (c)  Polyp- 
terus  (ganoid).  (c)  Necturus  (primitive  salamander).  (e)  Dactylethra  (South 
African  frog),  (f)  Turtle. 

In  (a)  the  part  m  is  formed  by  a  fusion  of  the  anterior  rays.  The  pieces  kk, 
segmented  off  from  m  in  (b),  form  in  (c)  a  rhomboidal  plate.  In  (d)  this  plate 
has  grown  large  and  bears  a  pair  of  ossified  ilia,  i,  and  a  pair  of  centers  of  ossi- 
fication, the  ischia,  h.  In  (g)  appear  two  more  ossific  centers,  the  pubes,  g.  (f) 
is  a  typical  pelvic  girdle,  with  all  its  parts.  The  epipubis,  e  in  (e),  is  incidental 
and  unimportant  in  this  connection. 

no  solution  for  the  various  conditions  that  occur  in  ganoids, 
some  of  which,  at  least,  ought  to  be  included  in  the  line  of 
descent.  For  these  a  plausible  solution  is  offered  in  Fig.  44,  ^ 


THE   ENDOSKELETON  171 

although  here  the  fault  lies  in  the  derivation  of  ganoid  condi- 
tions directly  from  the  primitive  form  without  accounting 
for  that  of  the  selachians.  The  first  of  these  figures  is  that 
of  Aclpenser,  the  sturgeon  (}Fig.  44,  a),  and  still  represents 
the  primitive  condition  of  parallel  fin-rays,  save  that  several 
of  the  anterior  ones  have  fused  in  order  to  meet  the  greater 
strain  imposed  upon  them.  This  tendency  has  progressed  still 
farther  in  a  related  ganoid,  Scaphyrhynchus  (Fig.  44,  b), 
where  the  proximal  portions  of  nearly  all  the  rays  are  included 
in  the  heavy  basal  piece,  which  bears  both  the  distal  portions 
of  the  rays  of  which  it  is  composed  as  well  as  the  few  original 
rays  which  have  not  entered  into  its  formation.  In  this  an 
important  step  is  the  formation  of  a  pair  of  little  pieces,  which 
are  segmented  off  from  the  proximal  ends  of  the  two  basal 
pieces,  and  which  serve  to  interpret  the  condition  found  in 
Polyptcnis,  in  many  ways  the  highest  of  the  living  ganoids 
and  the  one  nearest  the  amphibians  (Fig.  44,  c).  On  this  the 
basal  piece  of  each  fin  has  partly  ossified  and  becomes  a  long 
limb-bone  highly  suggestive  of  a  femur,  and  the  two  are  at- 
tached to  a  small  mid-ventral  piece,  a  rudimentary  girdle, 
which  is  divided  by  a  suture  into  two  portions,  plainly  the  same 
as  the  two  small  inner  pieces  of  Scaphyrhynchus f  here  united 
to  form  a  rhomboid  plate. 

The  derivation  from  this  of  the  condition  found  in  the 
urodele  Necturus  becomes  at  once  evident  in  a  comparison 
of  the  two  (Fig.  44,  c  and  d),  in  the  latter  of  which  the 
steps  in  advance  consist  of  the  enlargement  of  the  plate,  its 
connection  with  the  vertebral  column  through  two  small 
processes,  the  ilia,  that  extend  dorsally,  and  the  appearance 
of  two  centers  of  ossification  posteriorly.  In  this  the  real 
transition  from  the  fish  type  to  that  characteristic  of  the  higher 
vertebrates  consists  of  the  direct  connection  between  the  hip- 
girdle  and  the  vertebral  column,  a  condition  never  found  in 
fishes.  That  this  attachment  has  been  newly  acquired  at  the 
stage  represented  by  Necturus  is  evidenced  by  the  lack  of 
difference  between  the  sacral  vertebra,  to  which  the  attachment 
is  made,  and  the  adjacent  ones,  as  well  as  by  the  frequency  of 


172  HISTORY   OF   THE    HUMAN    BODY 

variation  in  the  vertebra  selected  for  this  attachment,  as  ex- 
plained above. 

Within  the  plate  itself  are  two  ossifications,  which  represent 
the  first  appearance  of  the  ischiadic  bones,  destined  to  become 
an  important  element  in  the  hip-girdles  of  higher  forms;  and 
in  some  of  the  higher  amphibians,  another  pair  of  osseous  ele- 
ments, the  pubic  bones f  also  appear  in  much  the  same  con- 
dition (Fig.  44,  e). 

From  the  latter  form  of  girdle  to  that  of  a  reptile  (Fig. 
44,  f),  the  step  is  a  smaller  one,  the  main  difference  being  in 
the  formation  of  a  large  obturator  foramen  between  the  ven- 
tral elements,  pubis  and  ischium,  a  foramen  present,  though 
insignificant  in  Necturus  and  other  amphibia  (Fig.  44,  d  and 
e).  In  some  reptiles  the  obturator  foramina  become  con- 
fluent, forming  a  heart-shaped  foramen  cordiforme. 

Allowing  for  considerable  variation  in  form  and  proportion, 
the  pelvic  girdle  of  mammals  is  similar  to  that  of  reptiles, 
and  consists  of  the  three  elements,  ilium f  ischiunL_and-pubis. 
the  first  dorsal,  the  other  two  ventral.  The  ilium  is  attached 
to  the  sacrum,  which  varies  somewhat  in  the  number  of  ver- 
tebrae involved  in  its  formation,  and  the  ischium  and  pubis  of 
the  two  sides  usually  unite  in  the  mid-ventral  line  to  form 
a  symphysis,  although  in  man  the  symphysis  involves  the  pubic 
bones  alone,  the  ischia  being  wide  apart.  This  is  doubtless  in 
correlation  with  the  enormous  size  of  the  head  of  the  human 
infant,  for  the  passage  of  which  through  the  pelvic  outlet 
provision  must  be  made.  A  similar  case  is  found  in  birds, 
where,  with  the  single  exception  of  the  African  ostrich,  not 
the  ischia  alone,  but  the  pubes  also  are  wide  apart  to  allow 
for  the  passage  of  the  enormous  eggs,  characteristic  of  the 
Class,  and  out  of  all  proportion  to  anything  that  exists  else- 
where in  nature. 

The  history  of  the  gtinildfT-giV^10  differs  considerably  from 
the  foregoing  in  its  later  development  as  it  becomes  compli- 
cated by  the  addition  of  membrane  bones  from  without,  as  m 
the  case  of  the  skull.  Although  in  its  first  appearance,  in 
selachians,  it  differs  somewhat  in  form  from  the  hip-girdle, 


THE   ENDOSKELETON 


it  is  probable  that  its  early  history  is  similar,  and  that  it  was 
fomied_by  a  coalescence  of  basal  portions  nf  thAfin-r^y^  as  in 


_ 

the  ether  case,  its  size  and  shape  being  modified  in  accordance 
with  differences  in  function.  In  the  adult  selachians  it  ex- 
tends dorsal  to  the  insertion  of  the  fin  into  long  points,  and 
the  two  halves  become  distinct,  forming  a  symphysis  at  the 
mid-ventral  line,  thus  somewhat  resembling  a  pair  of  man- 
dibles. Each  lateral  piece  is  termed  scapulo-coracoid,  as  a 
suggestion  of  the  two  elements  to  which  it  is  to  give  rise, 


FIG.  45.     Shoulder-girdle  of  fishes. 

(a)  Dog-fish  (selachian).  (b)  Polypterus  (ganoid).  (c)  Cod  (teleost).  (d) 
Ceratodus  (dipnoan). 

s,  scapula;  ss,  suprascapula ;  c,  coracoid;  cv,  clavicle;  ct,  cleithrum;  xx,  supra- 
cleithra. 

but,  as  in  the  case  of  the  primordial  skull,  there  is  no  sug- 
gestion of  boundaries  between  the  two  portions,  although  it 
may  be  vaguely  indicated  by  the  point  of  insertion  of  the  fin, 
the  portion  dorsal  to  it  being  the  scapular  portion,  and  the 
other  the  coracoid. 

In  ganoids  there  becomes  associated  with  this  cartilaginous 
girdle  a  series  of  dermal  bones,  formed,  as  elsewhere,^  the 
fusion  of  scales.  There  are  two  of  these  bones  investing  each 
lateral  piece,  the  clayick^Jd&irig  ventral  and  the^oitot  lat- 
eral to  these.  Of  these  the  former  are  a  little  larger,  and 
form  the  symphysis  in  the  mid-ventral  line,  excluding  the 


174  HISTORY   OF   THE   HUMAN    BODY 

cartilage.  In  teleosts  the  cleithra  become  excessively  developed 
and  function  as  the  entire  girdle,  while  the  clavicles  are  want- 
ing and  the  cartilaginous  element  is  much  reduced.  There  are 
also  other  small  dorsal  pieces,  the  supra-cleithra  (post-tem- 
poral and  supra-clavide  of  many  authors),  which  connect  the 
girdle  with  the  skull. 

As  the  teleosts  represent  the  perfection  of  the  fish  type,  but 
are  not  in  the  direct  line  of  descent,  their  condition  represents 
a  specialization  not  closely  related  to  higher  forms,  and  the 
direct  history  is  carried  over,  with  a  small  interval,  from  the 
ganoids  to  the  amphibians.  In  these  latter  the  dermal  element 
is  but  little  in  evidence,  while  the  cartilaginous  part  is  volu- 
minous and  shows  centers  of  ossification.  In  urodeles  each 
half-girdle  is  in  the  form  of  a  thin  plate,  wrapped  about  the 
side  of  the  body  and  incompletely  divided  into  three  portions, 
a  dorsally  extended  scapula,  containing  an  ossified  area,  and 
two  ventral  extensions  separated  by  a  notch,  the  coracoid  and 
procoracoid,  both  cartilaginous.  There  are  here  no  dermal 
elements.  In  the  tailless  amphibians  the  cartilaginous  founda- 
tion is  much  the  same  as  in  urodeles,  but  there  is  also  an  ossi- 
fied area  in  the  coracoid,  and  a  dermal  clavicle,  in  the  form 
of  an  inverted  trough,  which  fits  closely  over  the  procoracoid, 
forming  a  compound  piece.  The  two  lateral  halves  become 
also  more  or  less  closely  associated  with  median  sternal  (and 
episternal)  elements,  and  in  the  more  specialized  frogs  the 
whole  forms  a  complicated  skeletal  armatuie  protecting  the 
vital  organs  and  forming  a  functional  thorax,  something  like 
that  of  higher  forms,  although  without  rib  components  and 
not  involved  in  the  respiratory  process. 

In  the  Amniota  there  appear  three  parts  to  the  girdle,  scap- 
ula, coracoid,  and  clavicle,  corresponding  in  the  main  to  those 
of  Amphibia.  The  two  first  are  preformed  in  cartilage  and 
ossify  later  on  in  development ;  but  the  clavicle  of  reptiles  and 
birds  has  no  cartilaginous  stage,  and  seems  thus  to  represent 
the  dermal  element  alone.  In  birds  the  two  clayiHps  fnsp  with 
a  median  inter ^7^^  (possibly  an  episternum),  to  form  the 
furgiila  or  "  wish-bone."  In  mammals  the  scapula  receives 
the  most  emphasis,  while  the  coracoid  is  never  present  as  a 


I 


THE    ENDOSKELETON 


175 


distinct  bone  save  in  the  monotremes.  In  others  it  ossifies 
from  a  separate  center,  but  soon  fuses  with  the  scapula  to  form 
the  coracoid  process.  The  clavicle  is  large  and  well  developed 
in  those  forms  in  which  strength  of  shoulder  is  especially  re- 
quired, as  in  most  cases  among  the  Rodentia,  Insectivora  and 


FIG.  46.     Anterior  fins  of  fishes. 

(A)       Dog-fish     (selachian).       (B)    Ceratodus     (Ji/moan). 

s,    scapula;    ss,    suprascapula;    c,    coracoid;    p,    propterygium ;    ms,    mesopterygium; 
In   B   there  are   radials  on  both   sides   of   a   central 


mt,   metapterygium;   rod,   radials. 
axis. 


Primates,  but  is  rudimentary  in  Carnivora,  and  is  entirely 
wanting  in  hoofed  mammals  and  in  Cetacea.  It  is  stated  by 
some  investigators  that  the  mammalian  clavicle  is  in  develop- 
ment a  compound  piece,  formed,  as  in  amphibians,  of  a  carti- 
laginous core,  overlaid  by  a  dermal  element.  If  this  be  so,  the 
former  may  be  the  procoracoid,  and^the  latter  the  true  clavicle, 
but  there  is  some  doubt  concerning  the  actual  conditions  of 
development,  and  the  whole  matter  needs  further  investigation. 
The  early  history  of  the  free  limbs  has  not  been  wholly 
deciphered.  The  fins  of  fishes  exhibit  a  great  variety  of  form, 
based  upon  a  series  of  fin-rays,  either  distinct  or  united  to 
basal  pieces.  Of  these  latter  the  posterior  limb  of  selachians 
shows  one,  the  basi-pterygium,  and  the  anterior  limb  three, 


176 


HISTORY   OF   THE   HUMAN    BODY 


named  in  order,  beginning  at  the  front,  pro-,  meso-,  and  meta- 
pterygium  (Fig.  46,  A).  These  latter  pieces  have  often  been 
considered  as  primitive,  and  various  attempts  have  been  made 
to  derive  from  them  the  long  bones  of  the  limbs  in  terrestrial 
forms;  but  they  are  not  always  found  in  ganoids,  which  ought 
to  show  here,  as  elsewhere,  transitions  to  the  higher  verte- 


Carpus 
[Tarsus 

Metacarpus    // 
[Metatarsus]  fj i 


II 


V 


IV 


V 
III  III 

FIG.  47.    Diagrams  of  typical  free  limb  skeleton. 

(A)  According  to  the  usual  nomenclature;  names  belonging  to  the  posterior  limb 
are  bracketed.  (B)  Suggestion  for  a  common  nomenclature  for  both  limbs. 

In  (A)  the  separate  carpal  and  tarsal  pieces  are  as  follows:  a,  radiale  [tibiale] ; 
b,  ulnare  [fibulare];  x,  intermedium;  y,  centrale;  i — 5,  carpalia  [tarsalia]. 

In  both   diagrams  the  more  constant  sesamoids  are  indicated  by  dotted  lines. 

brates.  The  Dipnoi  show  a  beautifully  symmetrical  type  of 
fin-skeleton,  which  consists  of  a  jointed  central  axis  with  rays 
upon  either  side,  a  type  which  many  have  regarded  as  the 
primitive  form  from  which  all  the  other  cases  have  been  de- 
rived, and  have  named  it  accordingly  the  archipterygium 
(Fig.  46,  B).  The  highly  specialized  Dipnoi,  however,  are 
not  the  proper  animals  to  which  to  look  for  primitive  condi- 


THE   ENDOSKELETON  177 

tions,  and  it  is  also  true  that  as  a  rule  it  is  unsymmetrical  and 
not  symmetrical  forms  that  show  the  early  conditions  of  a 
part. 

Immediately  above  the  fishes  the  chiropterygium  or  hand- 
form  appears,  a  type  so  definite  and  fixed  in  character  that  all 
limbs  from  the  amphibians  on  may  be  directly  referred  to  it, 
while  it  appears  in  almost  its  typical  condition  in  animals 
widely  separated.  (Fig.  47.)  "It  consists  of  a  proximal  joint 
formed  of  a  single  bone,  a  second  joint  of  two,  followed  by  a 
series  of  several  small  pieces  from  which  extend  five  digits, 
each  composed  of  several  bones.  Unfortunately  the  parts  were 
originally  named  without  much  reference  to  the  striking  cor- 
respondence (serial  homology)  between  the  anterior  and  pos- 
terior limbs,  and  thus  some  of  the  corresponding  parts  have 
received  very  different  names,  leading  to  a  redundancy  of 
terms.  The  nomenclature  and  correspondence  are  indicated 
in  the  following  table : 

Anterior  Limb.  Posterior  Limb. 

Humerus  Femur 

Ulna   (outer)    Fibula   (outer) 

Radius    (inner)    Tibia    (inner) 

Carpus    Tarsus 

Metacarpus    Metatarsus 

Phalanges Phalanges 

The  digits  are  designated  either  by  number,  I-V,  or  else 
the  Latin  names  for  the  fingers  are  used,  pollex,  index,  medius, 
annular  is,  minimus.  These  names  are  applied  equally  to  both 
members,  except  that  for  digit  I  of  the  posterior  limb  the  term 
hallux  is  employed  instead  of  pollex. 

The  nomenclature  of  the  bones  of  carpus  and  tarsus  has 
caused  by  far  the  most  difficulty,  as  they  are  extremely  vari- 
able and  liable  to  fuse  with  one  another  or  to  disappear.  Still, 
in  spite  of  much  irregularity,  they  are  reducible  to  a  type  or 
pattern,  as  given  in  the  diagram,  to  which  the  individual  cases 
may  be  referred.  In  this  typical  form  (Fig.  47,  A),  there  is 
a  piece  at  the  distal  end  of  each  of  the  two  limb  bones  of  the 
second  joint,  and  five  at  the  proximal  ends  of  the  five  meta- 


I78  HISTORY    OF    THE    HUMAN    BODY 

carpals  [or  metatarsals].  These  are  called  radiale  and  ulnare 
\_tibiale  and  fibulare~\,  and  the  five  carpalia  [or  tarsalia]  re- 
spectively. Of  the  latter  set  the  individual  bones  are  desig- 
nated by  a  number,  as  tar  sale  IV ,  car  pale  II,  car  pale  V,  etc. 
Aside  from  these  there  are  two  median  'pieces :  the  inter- 
medium, lying  between  the  two  proximal  elements,  and  the 
centrale,  distal  to  it  and  enclosed  by  both  rows. 

The  close  similarity  that  exists  between  the  anterior  and 
posterior  limb  skeleton,  and  the  frequency  with  which  the  two 
need  to  be  compared,  piece  by  piece,  leads  one  frequently  to 
wish  that  the  nomenclature  of  the  two  should  be  unified 
throughout,  as  has  already  been  done  in  part  in  the  distal  por- 
tion. Probably  the  chief  objection  to  this  lies  in  the  diverse 
ideas  which  still  exist  concerning  the  serial  homology  of  the 
parts  (treated  here  in  Chapter  VI),  yet  the  practical  advan- 
tage that  has  already  resulted  from  a  partial  uniform  nomen- 
clature in  carpus  and  tarsus  shows  the  possibility  of  com- 
pleting such  a  scheme  as  a  working  hypothesis,  without  rais- 
ing the  question  of  serial  homology.  Such  a  scheme  is  shown 
in  Fig.  47,  B,  which  may  be  compared  with  A  of  the  same 
figure,  that  shows  the  nomenclature  now  in  use. 

In  addition  to  the  definite  carpal  and  tarsal  bones,  which 
belong  to  the  primary  limb  skeleton,  there  are  certain  other 
elements  of  sporadic  occurrence,  that  are  situated  in  or  about 
the  tendons  of  muscles  and  serve  some  mechanical  purpose  in 
connection  with  the  action  of  those  parts.  These  are  scsamoid 
bones,  and  are  suggested  in  the  diagram  by  dotted  lines.  Of 
these  the  most  usual  are  a  radial  and  an  ulnar  one  [tibial  and 
fibular]  placed  upon  the  free  edges  of  the  carpus  [or  tarsus]. 
Sesamoids  also  occur  in  other  portions  of  the  limb  skeleton; 
thus  the  patella,  constantly  found  in  birds  and  mammals,  oc- 
curs in  the  posterior  limb  between  the  first  and  second  long 
joints,  and  forms  the  protuberance  at  the  knee.  Small  sesa- 
moids,  often  associated  in  pairs,  are  found  on  the  flexor  side 
of  the  digits  between  the  phalanges. 

Although  the  above  scheme  for  a  typical  carpus  or 
tarsus  and  its  nomenclature  seems  to  serve  the  purpose 
of  naming  the  parts  in  all  cases  (Fig.  48),  there  are  many 


THE    ENDOSKELETON 


179 


facts  which  forbid  us  from  imagining  that  it  is  really  a  primi- 
tive condition.  Thus  the  animal  in  which  it  comes  to  its  most 
perfect  realization  is  the  turtle,  the  carpus  of  which  is  almost 
diagrammatic,  while  the  salamanders,  where  a  more  primitive 
type  is  to  be  expected,  depart  almost  as  widely  from  the  dia- 
grams as  do  the  mammals.  From  certain  indications  it  seems 
probable  that  in  the  early  carpus  and  tarsus  there  were  two 
centralia  (Fig.  48,  a),  and  that  the  separate  bones  were  ar- 


in  iv 


FIG.  48.  Various  forms  of  carpus.  Figures  (a)-(c)  after  ELISA 
NORSA;  figure  (e),  after  FLOWER. 

(a)  Sphenodon  (Hatteria),  a  New  Zealand  lizard,  (b)  Chick  embryo,  early 
stage.  (c)  Chick  embryo,  later  stage,  (d)  Lacerta,  a  European  lizard,  (e)  Talpa, 
European  mole,  (f)  Pig. 

R,  radius;  U,  ulnar;  r,  radiale;  u,  ulnare;  *,  intermedium;  c,  centrale;  1-5,  car- 
pal ia;  p,  pisiforme;  f.  os  falciforme;  I-V  digits. 

ranged,  not  symmetrically,  but  in  oblique  rows  continued  more 
or  less  directly  to  the  digits,  and  suggesting  the  derivation  of 
both  carpus  [and  tarsus]  and  digits  from  long  fin-rays,  divided 
into  numerous  joints. 

In  the  nomenclature  of  the  carpal  and  tarsal  bones  em- 
ployed in  human  anatomy  we  have  an  unusually  good  illustra- 


i8o 


HISTORY   OF    THE   HUMAN   BODY 


tion  of  parts  named  with  reference  merely  to  a  single  animal 
form,  for  the  names  given  them,  e.g.,  cuneiform,  trapezium, 
multangulum  ma  jus,  os  magnum,  etc.,  are  wholly  relative  and 
might  not  apply  even  to  closely  allied  animals.  Since,  how- 
ever, these  older  names  are  still  more  or  less  used,  it  may  be 
well  to  compare  them  with  the  more  unusual  nomenclature 
given  above,  a  comparison  which  may  be  best  shown  in  the 
form  of  a  table  as  follows : 


OLDER  NOMENCLATURE 

MORPHOLOGICAL 
NAMES 

OLDER  NOMENCLATURE 

Hand 

Hand 

Foot 

Foot 

scaphoides  (naviculare)* 

radiale 

tibiale 

(  os  calcis  (calcaneus) 

J  astragalus  (talus) 
(  (value  undetermined) 

lunare  (lunatum) 

intermedium 

cuboides  (triquetrum) 

ulnare 

fibulare 

anlage  in  embryo, 
with  occasional 
persistence 

centrale 

naviculare 

trapezium  (mult- 
angulum majus) 

carpale  I 

tarsale  I 

entocuneiforme 

trapezoides  (mult- 
angulum minus) 

carpale  1  1 

tarsale  II 

mesocuneiforme 

os  magnum  (capi- 
ta turn) 

carpale  III 

tarsale  III 

ectocuneiforme 

unciforme  (hamatum) 

carpale  IV 
carpale  V 

tarsale  IV 
tarsale  V 

'os  cuboideum 

radial 
sesamoid 

tibial 
sesamoid 

pisiforme 

ulnar 
sesamoid 

fibular 

sesamoid 

*  Synonyms  used  more  frequently  by  European  anatomists,  are  given 
in  parentheses.  These  latter  have  been  adopted  by  the  BNA,  but  the 
substitution  of  them  in  America  for  the  more  familiar  terms  placed  first 
in  the  above  list,  will  be  difficult  to  accomplish,  and  it  is  a  question  if  it 
be  desirable,  since  neither  set  of  terms  rests  upon  a  morphological  basis. 


THE   ENDCSKELETON  181 

Aside  from  the  normal  bones  in  carpus  and  tarsus  there 
occur  occasionally  supernumerary  elements  of  greater  or  less 
frequency.  Thus  in  Man,  in  which  the  subject  has  been  nat- 
urally investigated  the  most  thoroughly,  there  have  been  re- 
corded for  the  carpus  fifteen  or  sixteen  such  elements,  aside 
from  the  occasional  occurrence  of  the  division  of  a  normal 
element  into  two  (bipartite).  The  summary  of  such  elements, 
so  far  as  known,  resting  upon  the  investigation  of  several 
thousand  human  carpi,  is  shown  in  Fig.  49.  Similar  super- 
numerary elements  occur  in  the  tarsus ;  the  most  important  of 


FIG.  49.  Diagrams  of  the  supernumerary  carpal  bones.  [After 
PFITZNER.] 

r,  radiale  externum  (constant  in  apes);  ce,  centrale;  I,  (epilunatum) ;  s,  hypo- 
lunatum;  c,  triquetrum  secundarium;  p,  epipyramis;  y,  praetrapezium ;  s,  styloideum; 
i,  parastyloideum;  e,  metastyloideum;  m,  capitatum  secondarium;  g,  ossiculum  Gru- 
beri;  h,  os  hamuh  proprius;  v,  os  Vesalianum;  ps,  pisiforme  secundarium. 

Of  the  normal  carpal  bones  the  scaphoides  and  the  cuboides  are  represented  as 
bipartite.  This  peculiarity  has  been  also  observed  in  the  lunare  and  the  trapezoides. 

which  are  the  trigonum,  associated  with  the  astragulus,  and 
present  in  8%  of  the  cases  studied;  the  tibiale  externum  (n- 
12%)  ;  the  peroneum  (8-9%)  ;  and  an  intermetatarseum  (8- 


182  HISTORY    OF    THE    HUMAN    BODY 

9%),  a  derivative,  sometimes  of  the  first,  sometimes  of  the 
second,  metatarsal. 

The  typical  number  of  digits  is  five,  but  this  number  is  fre- 
quently reduced  by  the  loss  of  digits  at  either  end  of  the  series. 
Instances  of  reduction,  usually  with  vestiges  of  the  missing 
digits,  are  of  frequent  occurrence  and  range  from  cases  with 
the  loss  of  the  first  alone,  as  in  certain  salamanders,  or  of  the 
last  alone,  as  in  the  feet  of  birds,  to  the  extreme  case  ex- 
hibited by  the  horse,  in  which  the  middle  digit  is  alone  de- 
veloped, accompanied  by  vestiges  of  II  and  IV. 

In  the  pig  and  the  ox  two  digits,  II  and  V,  considerably 
reduced  in  size,  are  set  behind  the  two  well  developed  ones, 
III  and  IV,  and  terminate  in  horny  spurs  that  do  not  touch  the 
ground.  Digit  I  is  entirely  wanting. 

The  occasional  occurrence  of  hyperdactylism,  or  cases  with 
supernumerary  digits,  in  all  groups  of  tetrapod  vertebrates, 
together  with  the  presence,  in  numerous  cases,  of  extra  bones, 
sesamoid  and  otherwise,  beyond  and  at  the  sides  of  the  true 
digits  (e.g.,  os  falciforme  in  Fig.  48,  e),  have  often  been  in- 
terpreted as  pointing  to  a  previous  condition  with  more  than 
five  digits,  a  condition  that  would  well  accord  with  the  de- 
rivation of  the  hand  from  a  fin,  since  in  most  cases  the  latter 
structures  possess  more  than  five  fin-rays.  These  phenomena 
are  not,  however,  so  interpreted  by  all  morphologists,  and  the 
subject  is  a  controversial  one  at  present.  Such  an  hypotheti- 
cal digit  placed  before  the  thumb  or  great  toe  is  called  a  prce- 
pollex  or  prce-hallux  respectively;  the  one  continuing  the 
series  beyond  the  fifth  is  a  post-minimus. 

The  chiridial  appendage  just  described,  although  it  forms 
the  universal  plan  upon  which  is  based  that  of  all  tetrapod 
vertebrates,  is  yet  capable  of  great  modification  and  has  en- 
abled its  various  possessors  to  adapt  it  to  a  great  variety  of 
uses.  The  detailed  consideration  of  this  belongs  to  the  field 
of  special  comparative  anatomy  and  does  not  come  within  the 
scope  of  this,  work,  yet  as  an  illustration  of  the  general  prin- 
ciple of  adaptation  a  few  cases  may  be  mentioned  which  show 
certain  of  the  modifications  acquired  in  the  successful  adap- 


THE   ENDOSKELETON 


183 


tation  of  the  free  limb  to  locomotion  in  the  two  difficult  ele- 
ments of  water  and  air  (Fig.  50).  The  chiridial  appendage 
(cheiropterygium)  is  primarily  a  walking  leg  (Fig.  50,  a),  de- 
signed to  aid  in  pushing  the  body  along  the  ground  or,  when 
extremely  developed,  to  support  the  body  above  the  surface 
and  assume  the  entire  function  of  locomotion.  In  becoming 


FIG.  50.    Modifications  of  the  fore  limb. 

(d)  Necturus,  a  primitive  salamander.  (b)  Ichthyosaurus,  an  extinct  marine 
lizard,  (c)  Globicephalus,  a  cetacean  (dolphin),  (d)  Pterodactyl,  an  extinct  flying 
reptile,  (e)  Bird,  (f)  Bat. 

Figure  (a)  represents  an  unmodified  limb  skeleton;  (b)  and  (c),  limbs  modified 
for  swimming;  (d),  (e),  and  (i),  those  modified  for  flight.  Designations  as  in  the 
previous  figure. 

modified  to  subserve  the  needs  of  an  aquatic  life  it  needs  to 
change  into  a  paddle,  which  it  does  by  elongating  the  digits, 
pressing  them  close  together,  and  surrounding  them  by  a  web 
of  integument.  The  digits  also  often  become  hyperphalangeal, 
that  is,  they  develop  an  unusual  number  of  phalanges,  as  is 
seen  both  in  the  marine  lizard  Ichthyosaurus  and  in  the  en- 
tirely unrelated  branch  represented  by  the  Cetacea.  [Fig. 
50;  compare  (b)  and  (c).] 

In  Ichthyosaurus  the  five  regular  digits  are  retained,  and 
there  is  also  a  row  of  small  bones  along  the  outer  edge,  con- 


184  HISTORY    OF    THE    HUMAN    BODY 

sidered  by  some  to  be  a  sixth  digit,  a  "  post-minimus."  It  is 
probably  a  row  of  sesamoids  employed  here  to  widen  the 
paddle  and  thus  increase  its  effectiveness.  In  the  cetaceans  the 
external  digits  suffer  some  reduction  while  the  two  middle 
ones,  II  and  III,  are  lengthened. 

In  adaptation  to  flight  the  chiridial  appendage  forms  a 
framework  for  a  thin  surface,  without  appreciable  weight,  and 
formed  either  of  integument  or  of  integumental  structures  of 
some  sort.  There  have  been  at  least  three  independent  and 
perfectly  successful  attempts  to  solve  this  most  difficult  me- 
chanical problem,  each  one  involving  profound  changes  in  the 
limb  skeleton.  In  the  extinct  pterodactyls  the  principal  modi- 
fication consisted  of  an  extreme  lengthening  of  the  little  finger 
to  form  a  framework  for  the  wing  membrane;  in  the  bats  a 
similar  result  has  been  attained  by  lengthening  all  the  digits 
except  the  pollex ;  and  in  the  birds,  where  the  development  of 
feathers  necessitates  the  formation  of  a  firm  framework  with- 
out motion  between  the  parts,  there  is  formed  a  bone  complex 
composed  of  carpal  and  metacarpal  elements  and  several 
phalanges.  [Fig.  50;  (d)  to  (f).] 

At  the  conclusion  of  this  subject  it  may  not  be  without  in- 
terest to  review  briefly  the  subject  of  the  transition  between 
the  two  types  of  free  limb,  the  ichthyopterygium  and  the 
cheiropterygium,  in  which,  although  as  yet  no  theory  has 
gained  general  credence,  or  has  even  passed  beyond  the  stage 
of  an  ingenious  speculation,  many  interesting  suggestions 
have  been  advanced,  some  of  which  may  be  near  the  truth,  as 
may  at  any  time  be  shown  by  the  discovery  of  the  fossil  re- 
mains of  transition  animals,  unknown  at  present. 

In  general  the  similarity  between  fin-rays  and  digits  is  seen 
by  everyone,  and  this  probable  homology  is  rendered  more 
likely  by  the  fact  that  the  bones  of  the  digits  in  animals  with 
the  hand-form  of  limb  are  preformed  in  cartilage,  thus  sug- 
gesting the  fin-rays  of  the  selachians  and  ganoids.  Both  are 
divided  by  joints  into  movable  segments;  both  are  supplied 
with  flexor  and  extensor  muscles ;  and  in  some  fishes  the  rays 
are  prolonged  by  the  addition  of  horn  threads,  in  structure 


THE    ENDOSKELETON  185 

and  position  recalling  the  claws  or  nails  of  the  hand-form,  al- 
though placed  on  both  sides  instead  of  one.  Even  the  usually 
excessive  number  of  the  fin-rays  is  no  real  objection,  since  in 
some  fishes  this  number  is  a  very  limited  one,  and  the  theories 
of  prse-pollex  and  post-minimus  point  to  a  previous  larger 
number.  Thus  in  a  way  the  derivation  of  hand  or  foot  from 
the  fish-fin  may  be  accounted  for ;  but  an  insuperable  difficulty 
lies  in  the  presence  of  the  two  lengths  of  limb  bones  inter- 
posed between  the  girdle  and  the  distal  complex,  which  seem 
to  correspond  with  nothing  found  in  the  fin.  It  is  to  be  re- 
membered, however,  that  these  are  not  especially  long  in  the 
more  primitive  forms,  like  salamanders,  so  that  the  main 
obstacle  lies  not  so  much  in  their  length  as  in  their  very 
existence,  which  has  never  received  a  satisfactory  explanation. 

An  early  suggestion  along  this  line  was  based  upon  the 
anterior  fin  of  selachians  with  its  three  basal  pieces  (Fig. 
51,  a).  The  mesopterygium  is  usually  much  the  largest  and 
shows  a  tendency  to  form  the  sole  connection  with  the  shoul- 
der-girdle. If  this  becomes  established,  the  mesopterygium  be- 
comes  pJtfji*La*uei  and  the  pro-  and  metafirerpquun.  slipping 
away  from  the  girdle,  and  bearing  the  free  rays,  would  become 
respectively  radius  and  ulna  (Fig.  51,  b  and  c).  This  theory 
seemed  for  a  time  to  receive  especial  corroboration  from  the 
structure  of  the  paddle  of  the  extinct  sea-lizards,  Ichthyosaurus 
and  Pleisiosaurus,  but  the  time  is  now  passed  for  drawing 
such  broad  conclusions  from  a  chance  resemblance  in  some 
highly  specialized  form,  aside  from  which  it  must  be  noted 
that  the  theory  is  based  upon  the  anterior  fin  alone,  leaving 
the  more  primitive  posterior  one  out  of  the  question. 

When,  a  little  after  this,  the  biserial  dipnoid  fin  was  heralded 
as  the  primitive  type,  and  named  in  consequence  the  "  archi- 
pterygium,"  it  turned  thought  in  a  new  direction,  and  an  effort 
was  made  to  seek  in  the  more  primitive  cases  of  the  hand-form 
a  central  axis  with  lateral  rays  proceeding  from  it.  In  one 
such  attempt  the  central  axis  was  formed  by  humerus,  ulna, 
ulnare,  carpale  V  and  the  fifth  digit,  to  which  the  remaining 
bones  served  as  lateral  rays  upon  the  inner  or  radial  side, 


i86 


HISTORY    OF   THE    HUMAN    BODY 


thus  forming  a  uniserial  form,  as  in  the  selachians ;  in  another 
the  hind  limb  of  the  salamander  Ranodon  was  used  as  a  basis, 


Radius 


Radius 


Ulna 


[Ulna] 


[Radius] 


FIG.  51-  Two  theories  of  derivation  of  the  chiridittm  (cheiroptery- 
gium)  from  the  fin  (ichthyopterygium).  The  figures  represent  the  ante- 
rior limb  in  all  cases,  [a-c,  after  HUXLEY;  d-f,  after  POLLARD.] 

(a)  Cestracion,  a  primitive  selachian,  (b)  Ichthyosaurus,  an  extinct  lizard.  (c) 
Necturus,  a  primitive  salamander,  (d)  Chlamydoselachus,  a  primitive  selachian,  (e) 
Polypterus,  a  ganoid,  (f)  Ranodon,  a  slamander. 

In  the  three  upper  figures  the  ulna  is  dotted,  the  radius  designated  by  a  diagonal 
striping;  in  the  three  lower  the  reverse  is  the  case. 

and  femur,  fibula,  intermedium,  the  two  centralia,  carpale  II 
and  the  second  digit  were  taken  as  the  central  axis,  leaving  one 


THE    ENDOSKELETON  187 

digit  upon  the  inner,  and  three  upon  the  outer  side  to  serve  as 
lateral  rays.  Aside  from  the  unsafe  basis,  however,  upon 
which  all  such  theories  are  founded,  their  use  as  an  argument 
is  nullified  by  the  ease  with  which,  in  turn,  each  digit,  with  its 
associated  bones,  may  serve  either  as  a  hypothetical  central 
axis  or  as  a  lateral  ray. 

As  opposed  to  the  theories  thus  far  recorded,  which  seek 
to  derive  the  entire  free  limb  from  the  fin,  is  a  more  recent 
one,  based  upon  the  anterior  fin  of  the  ganoid  Polypterus,  in 
which  ulna  and  radius  are  derived  from  the  fin,  while  the 
humerus  represents  an  element  to  be  detached  later  from  the 
shoulder-girdle.  In  this  fin  (Fig.  51,  ft),  the  pro-  and  meta- 
pterygium,  already  in  the  form  of  long  bones,  articulate  with 
a  projecting  point  of  the  girdle,  while  the  meso-pterygium, 
reduced  to  a  small,  disc-shaped  element,  lies  between  them 
but  detached  from  both.  //  now  the  long  pro-  and  meta-ptery-\ 
gia  are  taken  as  radius  and  ulna  respectively,  ignoring  tfie 
meso-pterygium  for  the  present,  the  projecting  process  of  the 
shoulder-girdle,  which  articulates  with  both,  and  which  might 
easily  become  detached  from  the  main  mass  if  of  functional\ 
advantage,  would  become  a  humerus.  The  mesopterygium  1 
would  thus  become  intermedium  or  intermedium  and  centrale,  \ 
leaving  the  remaining  parts  of  the  carpus,  the  metacarpus  and 
phalanges,  to  be  formed  from  fin-rays.  This  is  in  many  ways 
the  most  satisfactory  solution  thus  far,  and  the  fact  that  it 
rests  upon  the  condition  found  in  a  single  species  is  no  real 
objection,  since  it  is  altogether  probable  that  terrestrial  verte- 
brates were  originally  derived  from  a  single  form,  perhaps 
a  single  species,  and  that  Polypterus,  with  its  close  anatomical 
correspondence  with  the  lowest  urodelous  amphibia,  is  nearly 
allied  to  that  form.  A  more  serious  objection  lies  in  the  fact 
that  the  posterior  fin  cannot  be  as  easily  developed  into  a 
cheiropterygium  as  can  the  anterior  one ;  still  the  anterior  and 
posterior  limbs  may  not  have  had  exactly  the  same  origin  or 
_^  early  history,  since  a  later  similarity  of  use  \vould  cause  a 
convergence  in  anatomical  structure.  Moreover,  as  a  mat- 
ter of  fact,  the  pelvic  fin  of  Polypterus  exhibits  proximally 


188  HISTORY   OF   THE    HUMAN    BODY 

a  single  long  bone,  showing  at  least  a  tendency  in  a  similar 
direction.  Here  the  matter  must  rest  for  the  present  from  lack 
of  further  evidence,  but  the  solution  may  be  found  at  any 
time,  as  has  happened  in  so  many  other  cases,  through  the 
discovery  of  the  fossil  remains  of  some  transition  form.  The 
pick  of  the  geologist  may  unearth  the  key  to  this  problem,  a 
true  palseontological  Rosetta  stone,  by  the  aid  of  which  the 
discrepant  records  written  on  fin  and  foot  may  be  deciphered 
and  brought  into  harmony.* 

*  Within  the  past  few  years  a  number  of  fossil  Polypteri  have  been  dis- 
covered in  the  Triassic  beds  along  the  shores  of  Lake  Tanganyika,  in  the 
waters  of  which  their  living  descendants  are  still  found.  This  would  seem 
a  favorable  locality  from  which  to  expect  results. 


CHAPTER   VI 
THE    MUSCULAR    SYSTEM 

"  When  we  no  longer  look  at  an  organic  being  as  a 
savage  looks  at  a  ship,  as  something  wholly  beyond 
his  comprehension;  when  we  regard  every  produc- 
tion of  nature  as  one  which  has  had  a  long  his- 
tory; when  we  contemplate  every  complex  structure 
and  instinct  as  the  summing  up  of  many  contrivances, 
each  useful  to  the  possessor,  in  the  same  way  as  any 
great  mechanical  invention  is  the  summing  up  of  the 
labor,  the  experience,  the  reason,  and  even  the 
blunders  of  numerous  workmen ;  when  we  thus  view 
each  organic  being,  how  far  more  interesting  .  .  . 
does  the  study  of  natural  history  become ! " 

CH.  DARWIN,  Origin  of  Species.    Chapter  XV. 

VERTEBRATES  possess  two  sorts  of  muscular  tissue,  un- 
striated,  or  involuntary,  in  the  form  of  cells,  and  striated  or 
voluntary,  in  the  form  of  long  fibers  usually  formed  from 
cell-complexes.  The  cells  of  the  involuntary  type  are  mesen- 
chymatous  in  origin  and  are  usually  associated  together  to 
form  layers  that  supply  the  walls  of  certain  internal  organs  and 
the  larger  blood  vessels  with  the  power  of  expansion  and  con- 
traction. All  voluntary  muscles,  on  the  other  hand,  are  de- 
rived directly  from  the  mesoderm,  and  the  ultimate  contractile 
organs  are  here  not  the  cells  themselves,  but  long  fibrils  of 
contractile  substance  built  up  by  the  cells,  or  often  by  long 
rows  of  cells,  and  organically  connected  with  them.  The  mus- 
culature of  the  heart,  in  some  characteristics  seemingly  inter- 
mediate between  the  two  classes  of  muscular  tissue,  is  a  modi- 
fication of  the  first  or  involuntary  type,  the  cells  of  which 
possess  a  peculiar  shape  and  become  marked  with  striae.  This 
derivation  is  rendered  clear  through  the  embryological  develop- 
ment of  the  heart,  which  is  seen  to  be  originally  an  expanded 
blood  vessel  with  an  hypertrophied  muscular  coat. 

189 


HISTORY   OF   THE   HUMAN    BODY 

The  muscles  of  the  second  type  are  in  general  under  the 
control  of  the  animal's  will,  and  are  thus  termed  voluntary, 
although  there  are  regions,  as  along  the  pharynx  and  oesopha- 
gus, where  genuine  striated  muscle  may  become  involuntary 
and  depend  for  its  action  upon  external  stimuli. 

The  striated  muscles,  which  form  the  subject  of  this  chap- 
ter, fall  into  three  anatomical  divisions  corresponding  to  those 
of  the  skeleton,  the  axial  muscles,  the  appendicular  muscles, 
and  the  visceral  muscles.  In  the  embryo  the  jijDpendicular 
muscles  are  derived  directly  from  the  axial,  both  arising  from 
the  dorsal  portion  of  the  mesodermic  somites,  the  epimeres; 
while  the  visceral  muscles,  limited  to  the  anterior  part  of  the 
body,  arise  from  the  ventral  portion,  the  hypomeres,  the  ele- 
ment which  in  the  remainder  of  the  body  develops  into  the 
pleuro-peritoneum,  and  furnishes  no  voluntary  muscles.  This 
difference  in  origin  sharply  divides  the  voluntary  musculature 
into  two  fundamental  groups,  (i)  the  parietal,  arising  from 
the  epimeres  and  including  the  axial  and  appendicular  mus- 
culatures, and  (2)  the  visceral,  arising  from  the  hypomeres, 
and  limited  to  the  anterior  part  of  the  body. 

Aside  from  these  three  primary  divisions  of  striated  mus- 
cles, there  occur  in  almost  all  vertebrates  certain  superficial 
muscular  elements,  usually  in  the  form  of  subcutaneous  layers 
and  intimately  associated  with  the  corium.  These  have  often 
been  treated  as  a  distinct  group  of  muscles,  but  recent  investi- 
gation has  placed  it  beyond  doubt  that  we  have  here  to  do  with 
muscular  elements  which  have  developed  independently  in  dif- 
ferent animals  in  response  to  certain  physiological  needs  and 
have  been  derived  from  the  most  convenient  subjacent  skeletal 
muscles,  whether  parietal  or  visceral.  However,  in  spite  of 
their  secondary  origin  from  other  muscular  groups,  they  have 
differentiated  so  far  structurally  that  it  is  far  more  convenient 
to  treat  them  as  a  distinct  group,  the  integumental  muscles, 
rather  than  to  consider  them  with  the  various  muscles  from 
which  they  were  originally  derived.  The  order  of  treatment, 
therefore,  of  the  muscles,  as  described  in  this  chapter,  will 
follow  the  plan  just  outlined:  the  first  to  be  considered  will 


THE  MUSCULAR  SYSTEM 


191 


FIG.  52.  Necturus,  with  the  integument  removed,  showing  the  primi- 
tive myotomic  muscles  and  the  differentiation  in  the  regions  of  the  limbs 
and  the  visceral  skeleton. 


192  HISTORY   OF   THE    HUMAN    BODY 

be  the  parietal  or  epimeric  muscles,  consisting  of  (i)  the 
axial,  and  (2)  the  appendicular  musculature.  This  will  be 
followed  by  a  short  sketch  of  the  visceral  or  hypomeric  mus- 
cles; and,  lastly,  the  integumental  muscles,  a  secondary  sys- 
tem, will  be  considered. 

The  primary  groups  of  skeletal  muscles,  with  the  exception 
of  the  integumental,  can  be  well  shown  in  a  condition  ap- 
proaching the  primitive  one  by  carefully  removing  the  skin 
from  a  salamander  and  inspecting  the  muscles  as  they  lie  in 
their  natural  position  (Fig.  52).  In  this  preparation  the 
axial  muscles  form  the  bulk  of  the  muscular  system,  and  are 
seen  to  consist  of  a  series  of  muscle  somites,  or  myotomes, 
separated  by  thin  perpendicular  planes  of  connective  tissue, 
the  myocommata.  These  axial  muscles  are  divided  by  a 
horizontal  furrow,  which  runs  along  each  side,  into  dorsal  and 
ventral  masses,  a  distinction  which  is  of  fundamental  impor- 
tance, as  these  masses  are  innerved  respectively  from  the  dor- 
sal and  ventral  branches  of  the  spinal  nerves.  The  groove 
dividing  them  marks  the  place  of  the  lateral  line  of  fishes, 
in  which  are  located  a  row  of  specialized  sense  organs.  The 
axial  muscles  begin  at  the  base  of  the  skull  and  continue  to  the 
end  of  the  tail,  but  show  some  interruption  of  their  course  in 
two  places,  corresponding  to  the  attachments  of  the  two  pairs 
of  limbs.  That  of  the  anterior  limbs,  however,  is  seen  to  be 
more  superficial  in  character,  and  when  the  appendicular  mus- 
cles, that  spread  out  in  thin  fan-like  sheets  over  the  trunk 
myotomes,  are  removed,  the  myotomic  muscles  are  displayed 
in  an  almost  uninterrupted  sequence.  In  the  case  of  the  pos- 
terior limbs,  however,  the  skeletal  girdle  comes  to  lie  imbedded 
within  the  body  muscles,  and  this,  together  with  the  cloaca, 
causes  a  considerable  hiatus  in  the  sequence  of  the  myotomes 
on  the  ventral  side,  although  dorsally  the  sequence  is  unbroken. 
The  muscles  attached  to  the  free  limbs  and  their  girdles  form 
the  appendicular  group,  of  little  proportional  importance  here, 
but  destined  in  the  higher  vertebrates,  with  the  development 
of  larger  and  more  powerful  limbs,  to  assume  a  far  greater 
bulk,  and  in  some  cases  to  even  surpass  that  of  the  axial 


THE    MUSCULAR   SYSTEM  193 

muscles.  There  will  be  noticed  this  difference  in  the  proximal 
muscles  of  the  anterior  and  posterior  limbs,  that  while  those 
of  the  latter  arise  almost  exclusively  from  the  girdle  itself, 
those  of  the  anterior  limbs  are  spread  out  fan-like  over  the 
axial  myotomes,  and  arise  either  from  the  myocommata  or 
from  the  integument.  In  the  higher  forms  this  difference  re- 
ceives much  greater  emphasis,  and  in  both  birds  and  mammals 
the  appendicular  muscles  of  the  fore  limbs  completely  enwrap 
the  body  both  dorsally  and  ventrally,  thus  concealing  the  axial 
muscles  entirely,  from  the  hips  to  the  neck,  except  in  the  ven- 
tral abdominal  region. 

Corresponding  to  the  extensive  development  of  the  visceral 
skeleton  in  the  animal  under  consideration,  the  visceral  muscu- 
lature is  also  large  and  well  shown.  This  occupies  the  ven- 
tral and  lateral  regions  of  the  head  and  neck,  and  includes  the 
massive  muscles  of  the  mandible,  which  extend  over  the  top 
and  sides  of  the  skull.  In  the  higher  forms  these  muscles, 
with  the  exception  of  those  of  the  mandible,  lose  in  bulk,  but 
perhaps  gain  in  complexity,  supplying  tongue,  pharynx,  and 
the  laryngeal  region. 

Although  thus  far  the  facts  presented  rest  upon  a  secure 
morphological  basis,  and  although  it  is  comparatively  easy  to 
follow  the  fate  of  the  primary  muscle  masses  as  a  whole  in 
the  separate  vertebrate  Classes,  the  further  emphasis  of  the 
separate  units  which  differentiate  from  the  primary  masses, 
that  is  to  say,  the  homology  of  the  individual  muscles,  is  a  sub- 
ject fraught  with  especial  difficulties,  and  is  one  in  which  the 
ground  is  still  uncertain,  owing  to  the  lack  of  fixed  principles 
to  direct  the  investigation.  To  begin  with  the  axial  muscles 
in  their  undifferentiated  condition  as  a  series  of  similar  myo- 
tomes and  follow  out  the  various  fate  of  their  derivatives 
throughout  vertebrates  as  the  parts  become  modified  to  sub- 
serve countless  special  uses;  to  start  with  the  limb  muscles  in 
the  form  of  myotomic  buds  and  follow  the  transformation 
outwardly  expressed  by  the  varied  shape  of  fin,  wing  or  leg; 
and  finally  to  discover  a  fundamental  plan  in  the  complicated 
visceral  muscular  system  of  selachians,  and  carry  out  the 


I94  HISTORY    OF   THE    HUMAN    BODY 

history  of  these  elements  as  they  gradually  assume  control  of 
such  different  parts  as  the  jaws,  the  auditory  ossicles  and  the 
laryngeal  cartilages ;  such  would  be  the  history  of  the  muscu- 
lar system  as  it  may  sometime  be  written,  a  history  even  the 
outlines  of  which  are  in  many  places  still  waiting  to  be  es- 
tablished. 

•  A  great  barrier  in  the  way  of  morphological  study  of  the 
muscles  lies  in  the  fact  that  there  is  no  definite  criterion  of 
homology,  no  simple  way  of  absolutely  proving  that  a  given 
muscle  in  a  certain  animal  is  morphologically  the  same  as  a 
similarly  related  one  in  another  species.  This  can  be  proven 
in  a  fairly  satisfactory  way  by  tracing  the  race  history  through 
a  series  of  forms,  provided  that  no  extensive  hiatus  occurs  in 
the  series;  but  more  often  the  animals  to  be  considered  are 
isolated  forms,  separated  by  wide  gaps  from  their  nearest  liv- 
ing allies.  With  other  organs  the  embryological  record  fur- 
nishes valuable  clews  and  often  traces  a  continuous  history,  by 
the  aid  of  which  the  condition  in  isolated  adult  forms  may  be 
interpreted ;  but  here  not  only  are  the  embryological  conditions 
extremely  difficult  to  interpret,  but  in  the  majority  of  cases 
the  historic  stages  are  not  there,  and  the  final  condition  is  seen 
to  arise  suddenly  from  a  mass  of  apparently  undifferentiated 
cells. 

In  attempting  to  establish  a  basis  for  homology  it  is  to  be 
remembered  that  a  muscle  is  not  always  a  definite  organ  like  a 
bone  or  blood  vessel,  but  is  rather  a  mass  of  muscular  fibers  set 
apart  for  a  more  or  less  distinct  purpose  and  differentiated 
from  the  surrounding  muscle  masses  in  proportion  to  the  defi- 
niteness  and  precision  of  its  action.  One  muscle  may  be  en- 
tirely isolated  from  the  adjacent  fibers  by  a  firm  cover  of  con- 
nective tissue  and  provided  with  a  special  tendon  attached  to 
a  definite  skeletal  process ;  another  may  be  a  bundle  of  fibers 
but  partially  separated  from  a  larger  mass  and  acting  only  in 
connection  with  it.  Often,  too,  the  action  produced  by  a  mus- 
cle is  not  precise  enough  to  prevent  numerous  variations,  and 
thus  may  vary  considerably  in  different  individuals  of  a  single 
species  or  even  in  the  two  sides  of  the  same  individual.  It  is 


THE    MUSCULAR    SYSTEM  195 

thus  necessary  to  study  all  possible  individual  as  well  as  specific 
variations  of  a  muscle  or  a  muscle  group  before  laying  down 
the  outlines  of  its  history. 

When  the  study  of  comparative  myology  was  in  its  infancy, 
muscles  were  homologized  and  named  from  their  general 
location,  appearance  and  .use,  without  regard  to  their  develop- 
mental history,  a  method  which,  while  fairly  safe  when  ap- 
plied to  closely  allied  forms,  was  apt  to  be  very  misleading 
when  applied  to  animals  as  different,  for  example,  as  members 
of  the  different  vertebrate  Classes.  Thus,  if  the  starting  point 
were  the  human  subject,  as  was  the  former  universal  custom,  it 
would  be  quite  easy  to  recognize  such  a  muscle  as  the  deltoid 
in  the  apes  and  monkeys,  which  possess  prehensile  arms  of  a 
similar  shape  and  used  in  a  similar  way,  but  it  would  be  much 
more  difficult  to  determine  the  same  muscle  in  the  ox,  which 
uses  its  fore  legs  so  differently,  and  the  difficulty  might  be- 
come insurmountable  in  a  form  as  different  as  a  turtle  or  a 
frog. 

Somewhat  more  reliable  as  criterions  for  homology  than 
position  or  use  are  the  origin  and  insertion,  the  points  at  which 
the  muscle  fibers  are  attached,  although  these  are  altered  by 
increase  or  decrease  in  volume  or  by  a  slight  change  in  use; 
and,  if  the  change  is  marked,  may  attain  quite  different  re- 
lationships to  the  skeletal  parts.  There  is,  however,  some  dif- 
ference in  the  relative  value  of  these  two  points,  the  origin 
being  the  more  constant  in  certain  regions,  the  insertion  in 
others.  It  has  long  been  considered  an  axiom  of  comparative 
myology  to  give  the  credit  for  greater  constancy  to  the  origin 
in  all  cases,  but  in  the  muscles  of  the  appendicular  skeleton  the 
reverse  seems  to  be  the  case,  since  here  the  insertions  are  at 
the  distal  end  and  are  effected  through  narrow  tendons,  the 
precise  attachment  of  which  is  a  matter  of  great  importance 
in  the  action  of  the  muscle,  while  the  origins  occupy  large  and 
rather  indefinite  areas,  the  extent  of  which  is  relative  to  the 
degree  of  development  in  each  case. 

Undoubtedly  the  most  reliable  criterion  for  muscular  ho- 
mology is  that  based  upon  the  constant  relation  between  a  given 


196     •       HISTORY   OF   THE    HUMAN    BODY 

muscle  and  the  nerve  which  supplies  it,  since  the  nerve  which 
originally  supplies  a  given  primitive  element,  such  as  a  myo- 
tome  or  a  limb-element,  never  forsakes  it,  but  follows  it 
through  all  its  vicissitudes  and  continues  to  supply  its  deriva- 
tives, of  whatever  complexity  or  form,  through  all  the  changes 
of  relation,  which  are  often  very  great.  A  conspicuous  ex- 
ample of  this  is  the  facial  nerve  (Vllth  cranial),  which,  in 
spite  of  its  name,  is  not,  so  far  as  it  is  a  motor  nerve,  origi- 
nally associated  with  the  face  but  with  the  hyoid  region  of  the 
neck.  In  the  lower  mammals  this  nerve  supplies  an  integu- 
mental  muscle  covering  the  side  of  the  neck,  of  which  the 
human  platysma  is  a  remnant,  and  as  it  happens  that  in  higher 
forms  this  sheet  becomes  extended  over  the  face  and  differen- 
tiates into  the  various  slips  that  form  the  mimetic  musculature, 
its  nerve  follows  it,  multiplying  its  branches  in  strict  accord- 
ance with  the  growth  and  differentiation  of  the  muscle,  until 
it  covers  the  entire  face  with  its  ramification  and  earns  the 
name  of  facialis.  Another  branch  of  the  same  nerve  supplies 
the  digastric  muscle  of  the  mandible  in  the  lower  vertebrates 
(the  equivalent  of  the  posterior  belly  of  the  like-named  mus- 
cle of  mammals),  and  when  in  reptiles  a  small  slip  detaches 
itself  from  this  muscle  and  wanders  into  the  middle  ear  to 
become  the  stapedius,  a  minute  branch  of  the  nerve  in  question 
follows  it  to  its  ultimate  location  and  furnishes  it  with  its 
nerve  supply. 

Were  it  possible  to  follow  each  motor  nerve  -fiber  from  its 
origin  to  its  connection  with  its  muscle,  it  would  probably 
serve  as  an  absolute  criterion  for  muscular  homology,  but 
there  is  much  chance  of  error  in  the  fact  that  an  anatomical 
nerve  is  not  a  single  fiber,  but  a  bundle  of  them,  and  while 
each  fiber  is  presumably  constant  in  its  supply,  there  is  some 
variation  in  the  way  in  which  they  are  put  into  bundles,  so 
that  no  one  can  be  sure  that  a  given  nerve  is  always  quite 
homologous  with  one  in  a  like  location  in  another  animal. 
This  is  especially  true  of  the  innervation  of  the  limbs,  where 
the  nerve  supply,  after  proceeding  from  a  certain  fairly  definite 
number  of  nerve  roots,  passes  into  a  plexus,  in  which  the 


THE    MUSCULAR    SYSTEM  197 

fibers  become  divided  up  and  reunited  in  new  combinations; 
and  although  in  two  allied  forms  the  final  nerves  that  emanate 
from  the  plexus  may  be  constant  in  number  and  position,  no 
one  can  be  quite  sure  that  their  make-up  in  individual  fibers 
is  the  same,  and  indeed  it  is  more  than  likely  that  they  are 
not.  Furthermore,  in  cases  in  which  a  single  muscular  ele- 
ment has  differentiated  into  a  group  of  well-separated  muscles, 
each  with  a  specific  action,  all  will  be  supplied  by  branches 
of  the  same  nerve,  and  the  innervation  will  furnish  no  clew 
to  homology,  beyond  that  of  identifying  them  as  members 
of  the  same  limited  group. 

Thus  the  criterion  of  nerve  supply,  although  in  theory  an 
accurate  and  definite  method,  often  fails  in  its  application 
through  variation  in  the  make-up  of  the  separate  nerve  bun- 
dles, and  while  undoubtedly  the  best  criterion  we  possess,  it 
cannot  be  employed  in  all  cases.  It  is  the  most  reliable  in  its 
application  to  the  axial  muscles,  where  there  has  been  the  least 
amount  of  differentiation,  and  where  the  primitive  segmenta- 
tion is  still  evident  or  but  slightly  disguised ;  it  has  also  proven 
of  value  in  the  muscles  of  the  visceral  system,  especially  in  the 
case  of  fishes  and  amphibians,  but  in  such  cases  as  the  highly 
differentiated  limb  muscles,  this  method  is  difficult  of  appli- 
cation, and  cannot  be  followed  beyond  the  homologizing  of 
the  larger  groups.  Here  the  character  most  to  be  depended 
on  is  the  insertion,  since  these  are  in  most  cases  by  narrow 
tendons  and  hence  very  definite,  while  the  origins  spread  over 
a  greater  or  less  area  in  proportion  to  the  size  of  the  muscular 
"  belly,"  or  fleshy  mass,  and  are  thus  somewhat  inconstant  in 
individuals  of  the  same  species,  or  probably  in  the  same  indi- 
vidual at  different  periods  of  its  life. 

Although,  in  the  study  of  the  morphology  of  the  muscles, 
through  the  incompleteness  of  the  embryological  record  and 
the  technical  difficulties  in  the  way  of  examining  it,  reliance 
has  hitherto  been  placed  mainly  on  the  comparison  of  adult 
forms,  much  may  undoubtedly  be  learned  concerning  the  his- 
tory of  individual  muscles  and  muscle  groups  from  their  on- 
togeny, especially  in  those  cases  in  which  the  animal  passes 


198  HISTORY   OF    THE    HUMAN    BODY 

through  an  active  larval  period,  and  hence  exhibits  the  earlier 
stages  in  functional  activity  and  consequently  in  greater  com- 
pleteness. 

From  both  this  source  and  from  the  study  of  adult  com- 
parative anatomy  certain  principles  may  be  deduced  concern- 
ing the  formation  of  muscles,  some  of  which  may  be  noted 
here. 

I.  Separation  of  an  indifferent  mass  into  several  elements. 
This  may  be  done  in  several  ways : 

(a)  By  the  growth  of  process  from  the  surrounding  skeletal 
parts,  thus  furnishing  separate  points  of  origin  for  different 
bundles  of  fibers. 

(b)  By  the  growth  of  a  skeletal  process  across  the  fibers  of 
a  long,  band-like  muscle,  thus  cutting  it  in  two;  a  secondary 
segmentation. 

(c)  By  the  higher  specialization  of  the  parts  of  insertion, 
thus  causing  a  splitting  up  of  the  muscle  bundle. 

Illustrations  of  the  first  of  these  may  be  seen  in  the  develop- 
ment of  the  processes  on  the  vertebrae,  which  results  in  the 
breaking  up  of  the  indifferent  myotomes  into  the  extremely 
complex  slips  seen  in  the  muscles  of  the  vertebral  column  of 
higher  forms.  The  second  method  is  rare,  and  occurs  in  the 
case  of  one  of  the  hyoid  muscles  of  the  frog  (the  fourth 
petro-hyoideus) ,  and  in  two  of  the  occipito-cervical  muscles 
in  the  mammals  (obliqui  capitis,  cf.  Fig.  55).  The  third  case 
finds  a  complete  illustration  in  the  differentiation  of  the  digits 
in  mammals  and  the  splitting  up,  both  of  the  tendons  and  of 
the  bellies  of  the  common  flexors  and  extensors,  in  exact  ac- 
cordance with  the  degree  of  independent  action  of  the  separate 
digits. 

II.  Fusion  of  separate  elements. 

This  may  occur  in  the  case  of  the  degeneration  of  a  function, 
but  it  is  seen  in  a  progressive  instance  in  the  case  of  long 
muscles  derived  from  elements  taken  from  separate  myotomes. 
Thus  the  external  oblique  muscle  of  the  abdomen  is  a  meta- 
meric  muscle  formed  by  contributions  from  a  series  of  suc- 
cessive myotomes.  In  salamanders  this  is  barely  differentiated 


THE    MUSCULAR   SYSTEM  199 

and  is  still  crossed  by  the  myocommata,  that  mark  the  original 
elements,  but  as  the  higher  forms  are  reached  the  myocommata 
disappear,  and  the  primary  segmental  nature  of  the  muscular 
sheet  is  shown  only  by  the  distribution  of  nerves  and  blood 
vessels.  In  the  rectus  abdominis  the  same  thing  has  taken 
place,  but  here  the  effacement  of  the  myocommata  is  not  com- 
plete, and  a  few  of  them  are  still  present,  even  in  mammals, 
forming  the  "  tendinous  interscriptions "  of  human  anatomy 
(3-4  in  man). 

III.  Extension  over  a  much  larger  area  of  an  element  origi- 
nally belonging  to  one  or  two  myotomes. 

This  is  seen  especially  well  in  the  case  of  certain  of  the 
muscles  of  the  anterior  limb,  notably  the  pectoralis  and  the 
latissimus  dorsi.  Small  and  not  very  extensive  in  the  earliest 
land  animals,  these  muscles  increase  with  the  importance  of  the 
limbs  to  which  they  belong,  and  may  eventually  extend  over 
a  large  number  of  myotomes.  In  their  final  condition  such 
muscles  closely  resemble  such  a  sheet  as  that  of  the  external 
oblique  of  the  abdomen,  but  their  composition  is  very  dif- 
ferent, the  one  being  the  result  of  the  fusion  of  elements  de- 
rived from  several  myotomes,  the  other  the  extension  of  an 
element  derived  from  one  or  a  few. 

IV.  Further  extension  of  a  pair  of  muscles  that  have  come 
to  meet  in  the  median  line  through  the  formation  of  a  median 
skeletal  crest  or  keel. 

The  two  most  conspicuous  examples  of  this  are  the  pector- 
alis muscle  on  the  ventral  side  of  the  thorax  and  the  tem- 
poralis  of  the  mandible.  Of  these,  one  is  originally  an  appen- 
dicular,  the  other  a  visceral  muscle,  each  probably  a  derivative 
of  a  single  myotome.  Both  extend  their  area  of  origin  with 
their  increase  of  function  and  size,  the  one  dorsally  over  the 
sides  and  top  of  the  skull,  the  other  ventrally  over  the  chest. 
Further  progress  being  stopped  by  the  meeting  of  the  two 
muscles  in  the  median  line,  they  obtain  an  extended  point  of  at- 
tachment through  the  formation  of  a  median  ridge.  The  most 
excessive  instance  of  this  is  seen  in  the  enormous  keel  of  the 
sternum  in  certain  birds,  notably  the  humming-bird. 


200  HISTORY   OF   THE    HUMAN    BODY 

Both  phylogenetically  and  ontogenetically  the  axial  muscles 
are  the  first  to  appear,  and  forcibly  suggest  some  lost  ancestor 
in  the  form  of  a  segmented  worm,  the  muscles  of  which  were 
mainly  longitudinal  in  direction  and  repeated  themselves  meta- 
merically.  The  appearance  of  these,  in  the  form  of  meso- 
dermic  somites,  is  one  of  the  earliest  post-gastrular  stages  in 
the  embryo,  and  is  the  first  suggestion  of  segmentation.  In 
the  lower  vertebrates  the  muscle  somites  develop  through  the 
longer  process  of  the  formation  and  extension  of  pairs  of 
lateral  diverticula  and  the  subsequent  separation  of  the  epi- 
meres,  as  in  the  theoretical  sketch  given  above  [Chap.  Ill], 
but  in  the  Sauropsida  and  Mammalia,  through  acceleration  of 
development,  the  epimeres.are  separated  from  an  indifferent 
mesoderm  in  the  form  of  approximately  cubical  blocks,  ar- 
ranged in  pairs  on  each  side  of  the  nerve-cord  and  notochord, 
and  were  long  considered  to  represent  "  primordial  vertebrae," 
a  name  occasionally  used  even  at  the  present  time,  although 
its  literal  meaning  has  been  long  since  discarded.  From  these 
the  myotomes  develop  through  the  formation  of  longitudinal 
fibers,  and  the  mesenchyma  of  the  intervals  between  the  meso- 
dermic  somites  becomes  transformed  into  the  myocommata, 
to  which,  on  either  side,  the  muscle  fibers  are  attached. 

There  is  thus  formed  the  primitive  system  of  axial  muscles, 
a  condition  which  is  retained  with  but  little  modification  in 
the  primarily  aquatic  vertebrates,  that  is,  in  Amphioxus,  cy- 
clostomes,  fishes  and  tailed  amphibians.  In  the  first  of  these 
the  myocommata  are  not  in  the  form  of  planes,  but  are  bent  in 
the  middle  at  an  acute  angle,  the  point  directed  forwards,  and 
are  set  into  one  another  so  that  several  consecutive  myocom- 
mata would  be  cut  in  a  cross-section  through  the  body  at  any 
plane.  In  many  fishes  the  myocommata  become  still  more  com- 
plex and  consist  of  several  pairs  of  cones,  those  of  successive 
myocommata  being  set  into  one  another  like  nested  cups.  This 
explains  at  once  the  shortness  of  fiber  of  fish  meat,  and  also  its 
curious  division  into  sets  of  concentric  circles,  when  cut  in 
cross  section,  two  phenomena  with  which  everyone  is  familiar. 
In  the  urodelous  amphibia  the  myocommata  are  almost  planes, 


THE   MUSCULAR   SYSTEM  201 

and  cut  the  body  nearly  at  right  angles  to  the  longitudinal 
axis,  thus  presenting  a  much  more  primitive  appearance  than 
that  found  in  most  fishes,  and  one  that  corresponds  closely  to 
the  embryonic  condition. 

Above  the  urodeles  the  development  of  more  massive  limbs 
and  limb-girdles,  the  same  element  that  produces  such  pro- 
found regional  differentiation  in  the  vertebral  column,  is  oper- 
ative here  also  in  modifying  the  simple  succession  of  typical 
myotomes,  a  condition  which  was  unaffected  by  the  delicate 
fins  and  weak  girdles  sufficient  for  their  primitive  aquatic 
environment.  The  hip-girdle  produces  the  most  direct  change 
through  the  intrusion  of  the  ilium  between  two  successive 
myotomes,  following  the  course  of  a  myocomma.  By  this  the 
axial  myotomes  become  divided  into  those  of  the  trunk  and 
those  of  the  tail,  and  the  gradual  increase  in  size  of  the  ilium 
and  its  extension  along  the  vertebral  column,  as  well  as  the 
formation  of  an  immovable  sacrum,  widens  the  space  between 
these  two  groups  of  muscles,  while  the  differentiation  of  func- 
tion between  trunk  and  tail  effects  profound  changes  in  the 
muscles  themselves.  The  principal  effect  of  the  shoulder-girdle 
in  this  regard  is  an  indirect  one,  for  while  it  presents  no  in- 
truding process,  as  in  the  case  of  the  ilium,  it  establishes  the 
anterior  point  of  support  and  causes  the  differentiation  of  the 
region  between  it  and  the  skull  into  a  neck,  which  becomes  in 
some  cases  extremely  mobile. 

In  most  reptiles  and  in  mammals,  however,  in  spite  of  these 
modifications,  the  succession  of  myotomes  remains  distinct, 
especially  dorsally,  and  through  all  the  regional  differentia- 
tions there  may  still  be  seen  the  segmental  character  of  the 
musculature;  but  birds  and  turtles  represent  two  types  of  ex- 
treme specialization  in  which  this  continuity  becomes  broken 
through  local  reduction.  The  cause  of  this  in  turtles  is  the 
formation  of  the  carapace,  in  which  all  free  movement  of  the 
vertebral  elements  and  ribs  is  lost  through  a  complete  anchy- 
losis, and  in  this  region  the  trunk  muscles,  though  laid  down 
in  the  embryo,  are  early  atrophied  and  become  lost.  The  only 
axial  muscles  retained  are  those  of  the  neck  and  tail,  together 


202  HISTORY    OF    THE    HUMAN    BODY 

with  a  powerful  retractor  system,  lying  on  the  ventral  side 
of  the  vertebral  centra  and  employed  in  drawing  back  the 
head  and  neck.  This  is  undoubtedly  homologous  with  the 
prevertebral  muscles  of  the  cervical  and  thoracic  region  in 
mammals  (longus  colli,  etc.),  and  belongs  with  the  ventral 
division,  innerved  by  ventral  branches  of  the  spinal  nerves. 
In  birds,  correlated  with  the  lack  of  mobility  of  the  trunk 
vertebrae,  the  corresponding  muscles  are  greatly  reduced,  but 
as  this  loss  of  motion  is  not  a  complete  one  as  in  the  turtle,  so 
also  is  the  reduction  of  the  muscles  not  as  extreme.  The 
muscles  of  neck  and  tail,  on  the  other  hand,  are  extremely  well 
developed,  thus  emphasizing  by  contrast  the  almost  rudimen- 
tary condition  of  the  muscles  of  the  back. 

In  taking  up  the  differentiation  of  the  axial  muscles  more 
in  detail,  their  division  into  dorsal  and  ventral  masses  must 
be  emphasized,  for,  although  both  are  derived  embryologically 
from  the  epimeres,  that  is,  the  originally  dorsal  portions  of 
the  mesodermic  somites,  yet  the  distinction  is  of  fundamental 
importance  topographically  and  morphologically,  because  they 
are  innerved  respectively  by  the  dorsal  and  ventral  branches  of 
the  spinal  nerves,  a  criterion  which  may  always  be  relied  upon 
to  help  out  the  homology  in  doubtful  cases,  where  a  muscle 
lies  near  the  boundary  between  them  or  where  an  extreme  de- 
gree of  differentiation  has  changed  the  primary  location. 
These  divisions  may  be  taken  up  in  order,  beginning  with  the 
ventral,  which  are  less  complicated  than  the  other. 

Throughout  the  course  of  these  ventral  axial  muscles  four 
distinct  regions  may  be  distinguished,  and  the  same  layer,  when 
it  is  possible  to  trace  it  from  one  of  these  regions  to  the  other, 
becomes  modified  to  partake  of  the  features  of  the  muscula- 
ture characteristic  of  each.  These  are  the' cervical,  thoracic, 
abdominal  and  caudal,  the  three  former  continuous,  the  lat- 
ter more  or  less  separated  by  the  interposition  of  the  hip- 
girdle  and  the  cloaca.  In  the  abdominal  region,  even  in  fishes, 
there  is  seen  a  tendency  for  the  musculature  to  break  up  into 
layers,  each  with  a  definite  direction  of  fibers,  distinct  from 
that  of  the  others.  In  amphibians  these  layers  consist  pri- 


THE    MUSCULAR    SYSTEM  203 

marily  of  an  obliquus  intermix,  an  obliquus  externus  derived 
from  the  former,  and  a  rectus  abdominis,  which  runs  along 
each  side  of  the  midventral  line  and  is  differentiated  from 
fibers  belonging  to  the  other  two.  To  these  are  added  during 
the  metamorphosis,  an  obliquus  externus  superficialis,  formed 
from  the  primary  externus,  and  a  transver sails,  differentiated 
from  the  primary  internus.  These  muscles,  with  the  well- 
known  differences  in  the  direction  of  their  fibers,  persist  in  all 
higher  vertebrates.  The  obliquus  externus  superncialis  of  the 
amphibians  is  no  longer  present  as  an  abdominal  muscle,  but 
seems  to  be  identical  with  the  two  serrati  posterior es,  superior 
and  inferior,  usually  treated  as  muscles  of  the  back  although 
innerved  by  ventral  nerves.  They  appear  in  rodents  and  in 
some  other  mammals  as  a  continuous  sheet,  but  in  man  the 
two  muscular  portions  (superior  and  inferior)  are  separated 
by  a  considerable  interval,  although  occasionally  connected 
by  a  thin  tendinous  sheet,  the  rudiment  of  the  intermediate 
portion. 

The  rectus  abdominis  is  the  only  one  that  retains  the  primi- 
tive longitudinal  direction  of  its  fibers,  and,  undoubtedly  cor- 
related with  this,  is  another  prirnitive  character,  that  of  the 
persistence  of  some  of  its  original  myocommata,  the  "  ten- 
dinous interscriptions  "  of  human  anatomy.  Connected  with 
the  pubic  end  of  the  rectus  in  mammals  is  a  small  muscle,  of 
uncertain  occurrence  in  man,  the  pyramidalis.  This  muscle  is 
a  rudiment  of  the  pouch  muscle  of  marsupials,  in  which  animal 
it  is  extremely  well  developed,  extending  from  the  marsupial 
bones,  upon  which  it  arises,  as  far  anteriorly  as  the  sternum. 
The  cremaster  muscle  of  the  mammalian  scrotum,  noticed  more 
particularly  in  connection  with  the  decensus  testiculorum  in 
Chapter  IX,  is  a  derivative  of  the  obliquus  internus.  The 
qiiadratus  himborum  belongs  also  in  this  place,  although  its 
derivation  has  not  yet  been  worked  out.  ,  It  appears  first  as  a 
distinct  muscle  in  reptiles  and  birds,  where  it  is  represented 
by  a  few  fibers  associated  with  the  transversalis.  In  mammals 
it  is  large  and  important. 

The  essential  elements  of  the  abdominal  musculature  may  be 


' 


204  HISTORY    OF    THE    HUMAN    BODY 

followed  into  the  thoracic  region,  although  here  a  new  char- 
acter is  introduced  by  the  presence  of  the  ribs,  which  occupy 
the  place  of  the  myocommata,  of  which  they  are  direct  deriva- 
tives, and  thus  impart  to  the  layers  a  segmental  character. 
The  obliquus  exiernus  (profundus)  and  internus  are  thus  con- 
tinued forward  as  the  external  and  internal  intercostals  .respec- 
tively, and  the  transversalis  is  represented  by  the  transversus 
thoracis  (triangularis  sterni),  in  man  very  variable  in  extent. 
The  levatores  costarum  appear  as  a  new  element,  peculiar  to 
the  thorax,  but  they  undoubtedly  belong  with  the  external  in- 
tercostals and  the  external  oblique. 

In  the  cervical  region  the  reduction  of  both  ribs  and  body 
cavity  necessitates  another  series  of  modifications  of  the  origi- 
nal elements.  There  belong  here,  as  ventral  muscles  of  the 
axial  system,  the  scaleni,  and  the  muscles  of  the  prevertebral 
group.  Of  these  the  scalenus  posterior  is  a  continuation  of 
the  levatores  costarum,  and  the  other  two  scaleni,  anterior 
and  medius,  belong  to  the  system  of  the  intercostals  and  are 
attached  to  those  portions  of  the  transverse  processes  of  the 
cervical  vertebrae  which  are  in  reality  anchylosed  ribs.  In  the 
same  category  belong  the  short  lateral  muscle,  rectus  capitis 
later  alls,  and  the  "anterior"  (ventral)  series  of  intertrans- 
versarii,  which  are  innerved  from  ventral  branches  and  are 
also  attached  to  the  rib  elements  of  the  cervical  vertebrae. 
The  prevertebral  muscles,  lying  on  the  latero-ventral  side  of 
the  vertebrae,  consist  of  the  longus  colli,  and  its  anterior  ex- 
tension, longus  capitis  (rectus  capitis  anticus  major),  also  of 
the  little  rectus  capitis  anterior  (rectus  capitis  anticus  minor), 
which  extends  between  the  skull  and  the  atlas,  near  the  rectus 
lateralis.  The  history  and  serial  homology  of  these  latter 
muscles,  other  than  the  fact  that  they  belong  with  the  ventral 
axial  muscles,  is  not  known. 

Both  ventral  and  dorsal  axial  muscles  are  continued  beyond 
sacrum  and  cloaca  along  the  caudal  vertebrae,  where  their  de- 
velopment varies  as  much  as  does  the  tail  itself.  As  it  is  more 
convenient  to  treat  of  the  caudal  musculature  as  a  single  sub- 
ject, the  discussion  of  the  ventral  muscles  of  this  region  will 


THE    MUSCULAR    SYSTEM 


205 


be  postponed  until  later  when  both  dorsal  and  ventral  caudal 
muscles  \yill  be  considered  together. 

The  dorsal  axial  muscles  are  much  more  restricted  in  ex- 
tent than  are  the  ventral  ones,  and  are  mainly  confined  to  the 


lonpssimus    capilis 
[  trachelo -mastoid  ] 

Jon&issirous  cervicis 
[  tmnsversalis    colli 


ilio-costaJis  cervicis 
[cervical is  ascendens] 


iljo-costolis  lumtorum 
sacro-  lumbalis  ] 


FIG.  53.    Muscles  of  the  human  back. 

The  superficial  layers  belonging  to  the  appendicular  system  have  been  removed. 
Upon  the  right  are  shown  the  three  portions  of  the  ilio-costalis  system  externally, 
and  within  this  the  longissimus  dorsi.  Upon  the  left  below  is  the  multifidus;  and 
above,  the  remainder  of  the  longissimus  system. 


206  HISTORY    OF    THE    HUMAN    BODY 

triangular  prismatic  spaces  located  on  either  side  of  the  mid- 
dorsal  line,  between  the  spines  of  the  vertebrae  and  the  trans- 
verse processes  and  ribs ;  they  are,  however,  far  more  complex 
in  structure,  and  are  characterized  by  the  presence  of  almost 
numberless  tendons  that  repeat  one  another  metamerically  and 
are  attached  to  corresponding  parts  of  successive  vertebrae  or 
ribs.  The  muscular  elements  are  also  less  differentiated  from 
one  another  than  in  other  parts  of  the  system,  and  in  many 
cases  the  subdivision  into  separate  muscles  is  an  arbitrary  one. 

But  little  has  been  done  with  the  muscles  of  this  group  from 
the  standpoint  of  comparative  anatomy,  and  thus  their  mor- 
phological history  cannot  as  yet  be  attempted.  It  must  suf- 
fice here  to  treat  them  to  a  rapid  review  as  they  exist  in  man, 
with  some  attempt  at  a  morphological  arrangement  of  their 
several  elements,  after  which  may  be  considered  what  is  known 
of  their  history. 

In  man  the  back  is  covered  superficially  with  several  layers 
of  muscles  which  are  dorsal  in  a  topographical  sense  only, 
having  secondarily  invaded  this  territory  from  other  places 
of  origin.  The  most  superficial  of  these,  trapezius  and  latissi- 
mus  dorsi,  form  a  thin  covering  over  almost  the  entire  back, 
effectually  concealing  all  beneath  them.  These  belong  to  the 
appendicular  system  and  appear  in  the  urodeles  as  small,  fan- 
shaped  muscles  which  extend  dorsally  from  shoulder  and 
humerus  respectively,  and  cover  but  a  small  portion  of  the 
trunk  myotomes.  With  the  gradual  increase  in  the  size  and 
importance  of  the  limbs  which  they  supply  they  have  enlarged 
to  the  extent  found  in  man  and  in  most  of  the  mammals.  The 
rhomboidei,  lying  beneath  these  and  of  less  extent,  likewise 
belong  to  the  appendicular  group.  Still  deeper,  beneath  all 
of  the  appendicular  muscles,  are  the  two  serrati  posterior  es,  the 
remains  of  a  continuous  sheet  in  certain  primitive  mammals. 
Their  innervation  from  ventral  branches  of  the  spinal  nerves 
shows  that  they,  too,  are  originally  strangers  to  the  dorsal 
region  and  belong  rather  with  the  ventral  axial  muscles,  under 
which  head  they  have  already  received  treatment. 

The  removal  of  all  the  above  exposes  the  genuine  dorsal 
axial  muscles,  the  derivatives  of  the  trunk  myotomes,  the 


THE    MUSCULAR    SYSTEM  207 

system  innerved  by  the  dorsal  branches  of  the  spinal  nerves. 
They  lie  lodged  in  the  space  embraced  by  the  spinous  and  trans- 
verse processes  of  the  vertebrae  and  run  in  the  main  in  a  longi- 
tudinal direction.  They  are  beset  with  metameric  series  of 
tendons,  which  attach  themselves  to  the  corresponding  portions 
of  successive  vertebrae  or  ribs,  and  plainly  suggest  a  segmental 
origin.  Proceeding  inwards  from  the  more  superficial  series 
they  may  be  divided  as  follows,  although  it  must  be  remem- 
bered that  the  muscles  are  often  closely  attached  to  one  another 
and  are  not  as  distinct  as  in  most  other  regions. 

I.  SPINO-TRANSVERSALIS  SYSTEM. — This  system,  the  fibers 
of  which  arise  from  spinous  and  insert  on  transverse  processes, 
consists  of  a  single  muscle,  the  splenius,  confined  to  the  anterior 
portion  of  the  trunk,  and  unrepresented  posterior  to  about  the 
middle  of  the  thoracic  vertebrae.     In  form  it  is  a  thin  sheet  of 
oblique  fibers,  a  portion  of  which,  splenins  cervicis  [colli], 
inserts  into  the  transverse  processes  of  cervical  vertebrae,  while 
the  remainder,  splenius  capitis,  inserts  into  the  base  of  the 
skull. 

II.  SACRO  -  TRANSVERSO  -  TRANSVERSALIS     SYSTEM.  This 

arises  from  a  large  mass  of  muscular  fibers  filling  in  the  space 
between  the  hip-girdle  and  the  most  posterior  ribs.  These 
fibers  take  their  origin  from  sacrum  and  ilium,  and  from  the 
lumbo-dorsal  fascia.  As  the  fibers  issuing  from  this  origin  are 
not  sufficient  to  supply  the  entire  vertebral  column,  they  are 
reinforced  by  others  which  arise  from  the  transverse  processes 
of  the  vertebrae,  beginning  with  the  lowest  lumbar.  The 
metameric  insertions'  are  in  two  longitudinal  ro\vs,  or  series, 
the  outer  into  the  ribs,  and  the  inner  into  the  transverse  pro- 
cesses. A  bundle  of  the  more  anterior  fibers  of  the  latter 
inserts  into  the  base  of  the  skull.  The  system  may  thus  be 
designated  sacro-transverso-transversalis,  the  first  two  elements 
designating  the  origin,  the  third,  taken  in  the  broad  sense  and 
including  the  ribs,  the  insertion. 

The  outer  series  begins  posteriorly  as  the  sacro-Iumbalis,  and 
becomes  continued  in  the  region  of  the  ribs  by  a  series  of  mus- 
cular slips  arising  from  lower  ribs  and  inserting  in  upper, 
miiscuhts  accessorius  ad  sacro-lumbalem.  A  still  further  con- 


208  HISTORY    OF    THE    HUMAN    BODY 

tinuation,  which  takes  the  series  into  the  cervical  region,  is 
formed  of  slips  that  arise  from  the  upper  ribs  and  insert  on  the 
transverse  processes  of  the  lower  cervical  vertebrse,  the 
cervicalis  ascendens.  As  these  three  muscles,  the  names  of 
which  have  come  down  to  us  from  premorphological  times,  are 
clearly  portions  of  a  single  series,  they  are  best  referred  to 
under  the  name  of  ilio-costalis  f  the  three  portions  being  dis- 
tinguished respectively  as  ilio-costalis  lurnborum,  ilio-costalis 
dor  si  and  ilio-costalis  cervicis.  [BNA.] 

The  inner  series,  posteriorly  blended  with  the  former,  ascends 
along  the  lumbar  and  lower  thoracic  regions  under  the  name  of 
longissimus  dor  si,  and  is  reinforced  anteriorly  by  the  trans- 
versalis  colli  and  the  trachelo-mastoid,  the  latter  inserting  on 
the  mastoid  process  of  the  skull.  In  placing  the  nomenclature 
on  a  morphological  basis  the  series  may  retain  the  name  of  its 
most  important  member,  the  longissimus,  the  parts  of  which 
are  longissimus  dorsi,  longissimus  cervicis  and  longissimus 
capitis.  [BNA.] 

In  tabular  form  the  parts  of  this  system  with  their  synonyms 
are  as  follows : 

Sacro-transverso-transversalis  System. 

OUTER  SERIES. 

ilio-costalis  lumborum    (sacro-lumbalis) 
ilio-costalis  dor  si   (accessorius  ad  sacro-lumbalem) 
ilio-costalis  cervicis  (cervicalis  ascendens} 

INNER  SERIES. 

longissimus  dorsi 

longissimus  cervicis  (transversalis  colli  s,  cervicis} 
longissimus  capitis  (trachelo-mastoideus.  s.  complexus 
minor) 

III.  SPINO-SPINALIS  SYSTEM. — This  is  a  single  series  lying 
beneath  the  preceding  close  to  the  median  line  and  consisting  of 
slips  that  arise  from  more  posterior  spinous  processes  and  insert 
on  more  anterior  ones,  skipping  at  least  one  vertebra  between 
origin  and  insertion.  This  series  is,  like  the  two  preceding, 
an  interrupted  one,  and  is  divided  into  two  constant  portions, 
spinalis  dorsi  and  spinalis  cervicis.  A  few  origins  from 


THE    MUSCULAR    SYSTEM 


209 


spinous  processes  of  cervical  vertebrae  and  associated  with  the 
semi-spinalis  of  the  next  system,  are  of  occasional  ocurrence, 
and  represent  a  spinalis  capitis. 


scmispinalis    /  complexus 
capitis       ^brventer  cervicis 


supraspinatus 
infraspinatus 
teres  minor 
teres  major 


pirifannis 
geraellus  superior 
obturator  interims 
e")  gemeuus  inferior 
quajiraros  femoris 


FIG.  54.    Muscles  of  the  human  back;  the  muscles  lying  beneath  those 
shown  in  Fig.  53. 
Certain  of  the  deeper  muscles  of  shoulder  and  hip   are  also  shown. 


210  HISTORY    OF    THE    HUMAN    BODY 

IV.  TRANSVERSO-SPINALIS  SYSTEM. — This  is  a  complex  sys- 
tem, the  elements  of  which  arise  from  transverse  processes  and 
insert  upon  the  spinous  processes  of  vertebrae  situated  more 
anteriorly.     It  is  in  close  contact  with  the  previous  system,  by 
which  it  is  covered,  and  may  be  imperfectly  divided  into  three 
series  or  layers,  superficial,  middle  and  deep,  called  respectively 
semi-spinalis,  niultifidus  and  rotatores.     These  layers  differ 
not  merely  in  position,  but  in  the  course  of  their  separate  slips, 
since  those  of  the  first  pass  over  4-6  vertebrae  between  origin 
and  insertion,  those  of  the  second  pass  over  2-3,  while  those 
of  the  deep  layers  are  themselves  subdivided  into  two  series, 
rotatores  longi  et  breves,  of  which  the  outer  pass  over  a  single 
vertebra,  while  the  inner  attach  to  adjacent  vertebrae. 

The  semi-spinalis  is  divisible  into  three  portions,  dorsi, 
cervicis  and  capitis.  The  last  of  these,  semi-spinalis  capitis,  is 
a  large  and  well-developed  muscle,  divided  longitudinally  into 
a  median  and  a  lateral  bundle ;  the  inner  one  of  these  is  tra- 
versed by  a  myocomma,  dividing  the  muscle  into  two  fleshy 
bellies,  from  which  comes  the  older  name  of  biventer  cervicis. 
The  outer  portion  forms  the  complexus  (major)  of  the  older 
anatomists. 

V.  INTERVERTEBRAL  SYSTEM.— Still  beneath  the  multifidus 
and  rotatores  are  several  series  of  short  muscles,  stretching  be- 
tween adjacent  vertebrae,   and  probably  representing  a   few 
fibers  of  the  primitive  myotomes,  which  have  remained  in  their 
original  condition.     Of  these  the  most  extensive  are  the  inter  - 
spinales,  lying  on  either  side  of  the  median  line,  and  extend- 
ing between  adjacent  spinous  processes.     Typically  associated 
with  all  the  intervertebral  intervals,  they  occur  in  man  mainly 
in  the  cervical  and  lumbar  regions,  including  the  first  and  last 
of  the  dorsal  vertebrae,  and  are  wanting  through  the  middle 
of  the  back.     A  second  set,  the  intertransversarii,  between  ad- 
jacent  transverse   processes,    occur   in    lumbar   and    cervical 
regions,  but  in  the  thoracic  are  represented  by  tendons  without 
contractile  fibers.     The  intertransversarii  of  the  lumbar  region 
are  divided  into  medial  and  lateral  portfons,  a  division  probably 
without  morphological  significance ;  on  the  other  hand,  those  of 


THE    MUSCULAR    SYSTEM 


211 


the  cervical  region  are  divided  the  other  way  into  ventral  and 
dorsal  portions,  which  belong  respectively  to  the  like-named 
divisions  of  the  axial  musculature  and  are  hence  morpholog- 
ically distinct.  The  ventral  cervical  inter transversarii  are  in- 
nerved  by  ventral  branches,  and  are  attached  to  the  ventral  or 
rib  element  of  the  complex  "  transverse  process  "  of  the  cervical 
vertebne.  They  are  thus  in  the  same  series  as  the  inter- 
costals,  and  have  been  already  treated  in  that  connection.  The 


FIG.   55.     Muscles  of  the  posterior  cervical  region    (human). 

rj,  rectus  capitis  posterior  major;  rn,  rectus  capitis  posterior  minor;  os,  obliquus 
capitis  superior;  oi,  obliquus  capitis  inferior,  rl,  rectus  lateralis.  Areas  of  origin  on 
the  skull  are  indicated  as  follows:  x,  splenius;  y,  semi-spinalis  colli;  s,  trapezius;  OS, 
obliquus  capitis  superior.  The  cervical  vertebrae  are  indicated  by  Roman  numerals. 

dorsal  cer^ncal  inter  transversarii,  on  the  other  hand,  are  in- 
nerved  by  dorsal  nerves  and  are  thus  serially  homologous  with 
both  portions  of  the  intertransversarii  of  the  lumbar  region  and 
with  the  tendons  which  have  replaced  them  in  the  thoracic 
region. 

Between  the  axis  and  the  skull,  corresponding  to  the  special- 
ized motions  needed  in  this  place,  there  has  developed  a  com- 
plex group  of  little  muscles,  which  are  seemingly  differentia- 


212  HISTORY   OF   THE    HUMAN    BODY 

tions  from  the  intervertebral  system.  The  rectus  capitis 
posterior  minor,  in  the  form  of  a  pair  of  small  slips,  extends 
between  the  rudimentary  spine  of  the  atlas  and  the  occipital 
bone,  and  a  similar  but  somewhat  larger  pair,  rectus  capitis 
posterior  major,  extends  from  the  spine  of  the  axis  to  the 
occipital  bone,  embracing  the  other  pair.,  An  obliquus  capitis 
inferior  extends  from  the  spine  of  the  axis  to  the  transverse 
process  of  the  atlas,  and  an  obliquus  capitis  superior  continues 
from  this  point  to  the  occipital  bone. 

This  entire  group  develops  from  what  appears  in  reptiles  as 
a  single  mass.  The  dorsal  branch  of  the  second  cervical  nerve 
runs  through  this  mass  and  divides  it  into  medial  and  lateral 
portions.  From  the  former  both  recti  (major  and  minor) 
develop,  through  a  longitudinal  separation  of  their  fibers,  and 
from  the  latter  arise  both  obliqui,  which  become  separated  from 
one  another  through  the  outward  growth  of  the  transverse 
process  of  the  atlas,  which  has  divided  the  muscle  across  its 
fibers,  a  perfect  example  of  the  principle  designated  above  as 
I  (&).,  p.  198. 

Posterior  to  the  pubo-ischiadic  symphysis  and  the  cloacal 
orifice,  structures  which  make  an  hiatus  in  the  ventral  series 
of  axial  muscles,  both  dorsal  and  .ventral  masses  become  re- 
duced in  size  and  taper  down  to  form  the  musculature  of  the 
tail.  But  little  morphological  research  has  been  devoted  to 
this  region,  and  even  in  mammals,  where  the  conditions  seem 
the  best  known,  there  is  much  to  be  done  to  complete  the  sub- 
ject. In  long-tailed  mammals  there  are  typically  two  exten- 
sores  caudcz  upon  each  side  of  the  mid-dorsal  line ;  an  extensor 
caudce  medialis,  which  is  a  direct  continuation  of  the  multifidus 
and  an  extensor  cauda  lateralis,  which  appears  between  multi- 
fidus and  longissimus,  but  does  not  seem  to  be  a  continuation  of 
either.  Upon  the  sides  are  two  abductores  caudce,  dorsalis 
and  ventralis,  the  former  being  short  and  of  lesser  functional 
importance,  the  latter  assuming  the  function  of  the  principal 
abductor.  Ventrally  there  is  a  single  pair  of  fiexorcs  (de- 
pressores)  cauda.  Of  the  above,  as  shown  by  the  innerva- 
tion,  the  extensors  and  dorsal  abductors  belong  to  the  dorsal 


THE    MUSCULAR   SYSTEM 


213 


system,  the  ventral  abductors  and  the  flexors  to  the  ventral. 
Aside  from  these  there  is  found  a  series  of  intertransversarii 
caudce,  presumably  belonging  to  the  dorsal  system  as  in  the 
case  of  the  lumbar  muscles  of  the  same  name. 


FIG.  56.     Human  caudal  muscles.     [After  LARTSCHNEIDER.] 

(A)  Ventral.     (B)  Dorsal. 

pir,  piriformis;  acv,  abductor  caudae  ventralis;  acd,  abductor  caudae  dorsalis;  eel, 
extensor  caudae  lateralis;  sea,  sacrococcygeus  anterior  (^rflexor  caudae).  The  lumbar 
vertebrae  are  designated  by  Roman  numerals,  the  coccygeal  by  Arabic. 


214  HISTORY    OF    THE    HUMAN    BODY 

In  the  tailless  apes  and  in  man,  corresponding  to  the  great 
reduction  of  caudal  (coccygeal)  vertebrae,  the  muscles  are 
greatly  reduced,  and  of  uncertain  occurrence,  yet  traces  of  all 
except  the  inter transversarii  have  been  detected,  and  certain  of 
them  are  fairly  constant.  The  sacro-coccygei  posteriores  (ex- 
tensores  coccygis)  are  found  upon  the  dorsal  side  of  sacrum 
and  coccyx,  and  in  individual  cases  may  represent  either  of  the 
two  extensors,  or  the  dorsal  abductor,  or  any  combination  of 
these.  Thus  in  100  human  bodies  the  medial  extensor  oc- 
curred 53  times,  the  lateral  extensor  43  times  and  the  dorsal 
abductor  87  times,  there  being  but  six  cases  in  which  indica- 
tions of  the  group  are  wholly  absent.  Upon  the  ventral  side 
of  the  coccyx  are  found  rudiments  of  the  ftexores  caudce,  de- 
scribed under  the  names  of  sacro-coccygei  anteriores,  s.  curva- 
tores  coccygis,  and  of  the  ventral  abductor,  here  called  the 
coccygeus  or  abductor  coccygis.  The  former  was  found  in 
102  out  of  1 10  cases,  the  latter  appears  to  be  constant,  although 
often  tendinous. 

Two  further  caudal  muscles,  pubo-coccygeus  and  ilio-coccy- 
geus,  are  found  in  long-tailed  mammals,  stretching  partly 
across  the  pelvic  floor,  and  sustaining  some  connection  with 
the  rectum  at  its  termination.  These  muscles,  although  rather 
small,  possess  much  morphological  interest,  since,  in  the  an- 
thropoid apes  and  in  man,  they  come  to  lie  transversely  across 
the  posterior  pelvic  opening  and  form  the  levator  ani,  a  muscle, 
which,  in  connection  with  the  coccygeus,  is  the  principal  ele- 
ment concerned  in  the  construction  of  the  so-called  diaphragnia 
pelvis.  This  is  a  muscular  and  tendinous  floor  or  partition, 
closing  the  posterior  outlet  of  the  pelvic  cavity,  and  its  forma- 
tion is  unquestionably  ai\  adaptation  to  the  erect  position  of 
the  anthropoids  and  man,  thus  strengthening  what  would 
otherwise  become  from  this  position  a  point  of  weakness. 

Although  the  exact  homologies  of  pubo-  and  ilio-coccygeus 
(levator  ani)  are  still  somewhat  obscure,  they  probably  belong 
to  the  ventral  axial  system,  and  are  not  connected  otherwise 
than  by  contiguity  with  the  other  muscles  of  the  perinseum, 
and  the  genital  organs,  such  as  the  sphincter  ani,  ischio-cav- 


THE    MUSCULAR   SYSTEM  215 

crnosus,  etc.,  which,  like  certain  of  the  pharyngeal  muscles,  are 
derivations  of  the  muscular  layer  of  the  intestinal  wall,  and 
belong  primarily  to  the  involuntary  system. 

Although,  as  previously  stated,  the  morphological  history  of 
the  dorsal  trunk  muscles  is  almost  unknown,  there  are  yet  a 
few  points  of  interest  which  may  prove  suggestive.  Among 
the  amphibians  there  is  a  suggestion  of  a  longitudinal  sub- 
division of  this  mass  into  a  medial  and  a  lateral  portion,  a 
change  which  in  the  reptiles  becomes  complete  and  definite.  In 
the  mammals  this  condition  is  still  evident,  with  but  a  few  sec- 
ondary modifications  which  tend  to  obscure  the  plan  somewhat. 
Thus  the  spino-spinalis  and  transverso-spinalis  systems,  the 
latter  with  all  of  its  subdivisions,  belong  to  the  medial  portion, 
while  the  sacro-transverso-transversalis  system,  on  the  other 
hand,  with  its  two  main  subdivisions  of  ilio-costalis  and  longis- 
simns, may  be  referred  to  the  lateral  portion.  Of  these  two 
systems  it  may  be  said  in  general  that  the  separate  slips  of  the 
medial  portion  are  inclined  towards  the  median  line  and  become 
inserted  into  spinous  processes,  while  those  of  the  lateral 
portion  are  inclined  outwards  (laterally)  and  become  inserted 
either  into  transverse  processes  or  into  ribs,  the  two  being 
genetically  the  same.  The  most  important  exception  to  this  is 
seen  in  the  longissimns,  which  does  not  indeed  violate  the 
principle  just  laid  down,  but  which  possesses  a  few  origins 
from  spinous  processes,  and  hence  from  the  median  line ;  these, 
however,  have  been  proven  to  be  of  secondary  origin,  arising 
through  an  association  of  a  part  of  the  longissimns  with  the 
spinalis,  a  relation  that  is  not  established  until  during  a  fairly 
late  embryonic  period.  The  only  other  doubtful  case  is  that  of 
the  splenius,  which  arises  from  spinous  processes  as  though 
belonging  to  the  medial  portion,  but  inserts  in  part  laterally. 
This  muscle,  however,  is  found  only  in  mammals,  and  may 
thus  be  considered  to  have  developed  long  after  the  division 
into  the  two  portions  had  become  established.  A  similar  rela- 
tionship in  the  obliqmts  capitis,  which,  although  belonging  to 
the  medial  portion,  has  its  origin  from  a  spinous  process  and  its 
insertion  into  a  transverse  process,  has  been  explained  above  as 


216  HISTORY   OF    THE    HUMAN    BODY 

a  secondary  condition  induced  by  an  extension  of  the  trans- 
verse process  of  the  atlas. 

To  complete  the  discussion  of  the  axial  muscles  there  still 
remains  a  group  to  be  considered,  and  that  is,  the  muscles  of 
the  eyeball,  for  the  proper  estimation  of  which  one  must  look 
to  embryology,  especially  that  of  selachians  and  amphibians. 
During  development  certain  pairs  of  mesodermic  somites  ap- 
pear in  the  head  as  well  as  throughout  the  trunk  and  tail,  and 
in  embryos  of  such  forms  as  those  named,  in  which  the  repe- 
tition of  the  early  stages  is  best  given,  these  epimeres  appear 
for  a  time  as  hollow  cavities,  the  so-called  "  head  cavities." 
With  several  of  these  head  cavities  dorsal  (motor)  elements 
from  the  cranial  nerves  become  associated,  and  although  cer- 
tain of  them  become  abortive,  after  continuing  up  to  the  stage 
of  possessing  a  nerve  supply,  three  pairs  remain,  those  asso- 
ciated with  the  third,  fourth  and  sixth  pairs  of  cranial  nerves 
respectively,  all  motor  nerves.  From  these  develop  the  muscles 
of  the  eyeball,  in  accordance  with  their  innervation;  that  is, 
from  the  walls  of  the  head  cavity  associated  with  the  third 
nerve  (motor  oculi)  develop  rectus  superior,  rectus  internus 
(medialis),  rectus  inferior  and  obliquus  inferior;  from  that  as- 
sociated with  the  fourth  nerve  (trochlearis)  there  develops 
the  obliquus  superior  alone ;  and  from  that  supplied  by  the  sixth 
nerve  (abducens)  arises  the  rectus  externus  (lateralis).  The 
retractor  bulbi,  a  muscle  which  appears  first  in  the  amphibians, 
develops  from  rectus  externus  and  is  consequently  innerved  by 
the  sixth  nerve ;  it  lies  enclosed  by  the  recti  and  often  develops 
into  a  hollow  cone  or  a  system  of  slips  which  act  almost  as 
independent  muscles.  It  is  well  developed  in  most  mammals, 
but  is  rudimentary  or  lacking  in  the  primates. 

Aside  from  the  head  cavities  from  which  the  muscles  of  the 
eyeball  are  derived,  a  matter  which  is  in  many  points  still 
controversial,  there  are  several  other  pairs  of  head  cavities 
which  come  to  nothing,  and  although  there  is  at  present  no 
generally  accepted  theory  concerning  the  primary  number  of 
cephalic  myotomes,  or  the  exact  relationship  of  those  that 
produce  the  eyeball  muscles,  it  is  evident  that  primarily  the 


THE    MUSCULAR    SYSTEM  217 

head  as  well  as  the  trunk  possessed  a  series  of  successive  my- 
otomes,  that  certain  of  those  furnished  materials  from  which 
the  eyeball  muscles  were  derived  and  that  all  other  muscular 
elements  thus  disappeared  at  a  very  early  period.  [Cf.  Fig. 
126,  A,  and  the  text  accompanying  it.] 

The  appendicular  muscles  are  the  direct  derivatives  of  cer- 
tain axial  metameres,  as  is  especially  well  shown  by  the 
ontogenetic  repetition  seen  in  selachian  embryos.  Here  the 
first  indication  of -a  lateral  fin  is  in  the  form  of  a  mass  of  tissue, 
appearing  externally  as  a  slight,  shelf-like  projection,  flattened 
dorso-ventrally.  That  this  may  be  considered  as  a  remnant  of 
a  once  continuous  fin-fold,  which  included  both  lateral  and 
median  fins,  has  been  explained  above  in  the  chapter  on  the 
endoskeleton.  Into  this  mass  there  grows  from  the  ventral 
portions  of  the  myotomes  a  series  of  what  have  been  aptly 
termed  myoiornic  buds,  becoming,  through  bifurcation,  two  for 
each  somite.  These  continue  their  development  until  the  fin 
is  supplied  with  a  segmental  series  of  muscle  bundles,  in  num- 
ber twice  that  of  the  somites  from  which  they  are  derived. 
The  actual  number  of  myotomes  thus  involved  differs  in  dif- 
ferent fishes,  but  in  all  there  are  certain  similar  principles  which 
may  be  brought  out  by  the  more  careful  study  of  the  case 
before  us.  By  comparing  the  three  diagrams  in  succession 
(Fig.  57)  it  will  be  noticed  that  of  the  contribution  of  the  first 
trunk  myotome  only  the  second  of  the  two  buds  succeeds  in 
developing,  and  that  that  one  becomes  abortive  and  fails  to 
furnish  a  permanent  contribution  to  the  fin.  Beyond  this  the 
contributions  appear  chronologically  in  regular  order  from  the 
front  backwards,  so  that  in  A  the  two  buds  of  the  seventh 
myotome  and  the  first  of  the  eighth  have  reached  the  fin,  while 
the  second  of  the  eighth  and  those  of  the  ninth  are  still  some 
distance  away  and  less  completely  developed.  The  second 
figure,  B,  shows  the  complete  disintegration  of  the  contribu- 
tion of  the  first  myotome  and  the  further  development  of  those 
from  the  second  to  the  sixth  inclusive,  that  is,  buds  II-XIII, 
while  buds  XIV-XVII,  or  those  of  myotomes  8  and  9,  have 
barely  reached  the  goal.  In  C  is  seen  a  single  bud 


218  HISTORY    OF    THE    HUMAN    BODY 

A 


(pectoral>    fin   in  the  dog-fish. 


-  which  the 


THE    MUSCULAR    SYSTEM  219 

belonging  to  the  tenth  myotome,  which  seems  here  far  from 
the  fin,  but  even  this  eventually  reaches  the  fin,  as  does  also  a 
second  one  from  the  same  myotome,  forming  buds  XVIII  and 
XIX  respectively. 

The  cause  of  this  apparent  struggle  to  reach  the  fin  on  the 
part  of  the  most  posterior  myotomic  buds,  is  one  which  ex- 
plains also  certain  other  characteristic  features  of  the  develop- 
ment, and  is  found  in  the  unequal  rate  of  growth  of  fin  anlage 
and  of  body  axis,  the  latter  considerably  surpassing  the  former. 
There  results  from  this  the  concentration  or  bunching  together 
of  the  nerves  of  the  free  limb,  especially  noticeable  in  Fig.  57, 
C,  a  circumstance  favorable  to  the  formation  of  a  nerve  plexus, 
and  as  this  concentration  of  a  number  of  pairs  of  nerves  to 
form  those  supplying  the  limbs  is  also  seen  in  the  case  of  all 
higher  vertebrates,  it  is  a  convincing  proof  of  the  derivation  of 
the  limb  muscles  from  a  more  extensive  series  of  myotomes 
than  that  indicated  by  the  adult  size  of  the  limb. 

From  this  sketch  of  the  development  of  a  limb  as  seen  in  the 
selachians  it  becomes  apparent  that,  were  it  possible  in  each 
group  of  vertebrates  to  trace  the  derivation  of  each  limb  muscle 
to  a  given  myotomic  bud,  or,  in  other  words,  were  it  possible 
to  follow  the  later  history  of  each  separate  myotomic  bud  to 
the  complex  conditions  of  higher  forms,  a  sure  and  certain 
homology  of  the  limb  muscles  could  be  carried  out ;  as  a  matter 
of  fact,  however,  the  primitive  history  in  the  development  of 
limb  muscles  is  found  only  in  fishes,  which,  in  their  adult  state, 
are  scarcely  beyond  the  last  of  the  three  stages  shown  here, 
while  in  all  higher  vertebrates,  from  the  urodeles  on,  these 
early  stages  are  dropped  out  completely,  and  in  a  developing 
limb,  in  which  for  a  time  the  cells  seem  exactly  alike,  and  with- 
out  differentiation  of  any  kind,  the  first  indication  of  any  defi- 
nite arrangement  is  the  collection  of  these  apparently  indif- 
ferent cells  into  masses  that  suggest  the  parts  as  they  exist  in 
the  adult. 

In  such  a  case,  then,  the  only  recourse  lies  in  the  comparison 
of  adult  forms,  and  here,  owing  to  the  complexity  of  the  subject 
and  the  technical  cifficulties  in  the  way  of  such  investigation, 


220  HISTORY    OF    THE    HUMAN    BODY 

much  remains  to  be  accomplished.  There  has,  indeed,  been  a 
large  amount  of  anatomical  work  done  on  the  subject,  but  little 
has  as  yet  been  attained  in  the  study  of  the  phylogenetic  devel- 
opment of  the  separate  muscles  or  muscle  groups,  and  the 
morphological  history  of  the  limb  muscles  is  as  yet  far  from 
complete. 

The  key  to  the  interpretation  of  the  muscles  associated  with 
the  hand  type  of  limb  (chiropterygium)  must  be  found,  if  at 
all,  among  the  tailed  amphibians,  which  are  the  first  animals 
to  possess  a  true  chiropterygium,  that  is,  a  free  appendage 
furnished  with  digits  instead  of  fin-rays,  and  here,  in  fact,  are 
found  many  highly  suggestive  conditions,  showing  many  of 
the  most  characteristic  muscles  of  the  higher  type  still  in  partial 
connection  with  the  myotomes  from  which  they  have  arisen. 
On  the  other  hand,  a  direct  comparison  of  these  muscles  with 
those  of  man  and  other  mammals  is  by  no  means  impossible 
and  yields  many  interesting  results,  since  the  distance  between 
these  two  groups  of  animals  is  much  less  than  is  commonly 
supposed,  and  the  intermediate  stages  do  not  include  either  the 
tailless  amphibians,  the  birds,  or  even  the  majority  of  reptiles, 
since  all  these  have  specialized  along  lateral  lines.  Indeed, 
man  himself  is  far  more  primitive  in  the  condition  of  his  limbs, 
with  their  ancient  inheritance  of  pentadactylism,  than  are 
either  the  salient  Anura,  with  their  four  anterior  digits  and 
their  specialized  hip-girdle,  or  such  reptiles  as  the  turtles,  in 
which  both  girdles  have  become  much  modified  in  connection 
with  the  formation  of  carapace  and  plastron. 

Probably  the  most  primitive  living  vertebrate,  above  the 
fishes,  is  a  large  aquatic  salamander,  Necturus,  generally  dis- 
tributed throughout  the  United  States,  except  the  Northeastern 
States,  and  the  extreme  South.  It  may  thus  be  assumed  to 
represent  in  the  muscles  of  its  free  limbs  the  earliest  condition 
of  chiropterygial  musculature  yet  remaining  to  us,  and  is  con- 
sequently of  the  utmost  importance  in  the  present  inquiry.  It 
will  be  remembered  that  in  most  vertebiates  there  exists  a 
certain  close  correspondence  in  the  skeletal  parts  of  anterior 
and  posterior  limbs,  a  so-called  serial  homilogy,  and  in  many 


THE    MUSCULAR    SYSTEM  221 

cases  this  correspondence  is  at  least  suggested  in  certain  details 
of  the  muscular  system.  Here,  in  Necturus,  however,  the 
correspondence  of  the  free  limbs  front  elbow  or  knee  down  is 
practically  an  exact  one,  and  includes,  not  only  the  skeletal 
parts,  but  the  muscles,  arteries  and  nerves,  precisely  what 
would  be  expected  in  this  primitive  form  if  the  serial  homology 
is  reall\  fundamental  and  not  due  to  secondary  modification 
through  a  similarity  of  use. 

Proximal  to  the  elbow  and  knee,  however,  there  is  little  if 
any  correspondence  or  even  similarity  in  the  musculature,  cor- 
responding in  this  respect  to  the  great  differences  in  the  two 
girdles,  and  therefore  for  descriptive  purposes  the  limb  may  be 
divided  into  unlike  proximal  portions,  to  be  treated  separately, 


FIG.   58.     Lateral  view  cf  shoulder  muscles  of  Necturus. 

Id,  latissimus  dorsi;  ds,  dorso-scapularis;  t,  trapezius;  /,  omohyoid;  k,  levator 
anguli  scapulae;  la,  levatores  arcuum;  d,  dorso-trachealis;  ph,  procoraco-humeralis;  as, 
anconeus  scapular  is;  al,  anconeus  lateralis. 

and  similar  distal  portions,  which  are  almost  identical  and  may 
thus  be  considered  together.  The  first  will  include  the  girdle 
and  the  proximal  joint  of  the  free  limb,  the  upper  arm  or  thigh, 
and  the  second  the  remainder,  or  that  from  elbow  or  knee  to 
the  end. 

The  principal  muscles  of  the  proximal  portion  of  the  anterior 
limb  in  Necturus  are  shown  in  figures  52  and  58.  Of  these 
certain  are  extrinsic  and  extend  from  trunk  or  head  to  the 
appendicular  skeleton ;  others  are  intrinsic,  both  origin  and  in- 
sertion being  upon  the  latter  part. 

_  Conspicuous  among  the  first  is  the  latissimus  dorsi,  certain 
parts  of  which  are  still  seen  to  arise  from  myocommata  in  the 
form  of  elements  in  the  ^act  of  separating  themselves  from  the 


222  HISTORY   OF   THE    HUMAN    BODY 

axial  musculature,  while  other  fibers,  the  anterior  portion,  no 
longer  show  their  segmental  origin.  Anterior  to  the  girdle 
lies  the  trapezius,  now,  like  the  anterior  part  of  the  latissimus, 
showing  no  trace  of  myotomic  origin,  but  undoubtedly  from 
that  source  originally.  Ventral  to  these  are  seen  two  slips 
clearly  derived  from  the  long  superficial  rectus,  and  still  farther 
ventral,  covering  the  chest  region,  lies  the  voluminous  pector- 
aliSj  still  in  part  composed  of  slips  attached  to  the  myocom- 
mata.  Beneath  these  superficial  layers  are  deeper  muscles,  like 
the  levator  scapula  and  the  serratus  magnus,  here  plainly  de- 
rived from  the  axial  muscles  in  the  form  of  separating  slips. 

Turning  to  the  intrinsic  muscles,  it  is  seen  that  the  outer 
surface  of  the  three  portions  of  the  girdle  is  covered  with  fan- 
shaped  sheets  that  converge  to  the  head  of  the  humerus,  where 
they  insert  near  together.  Of  these  the  dorsalis  scapulce  cov- 
ers the  scapula,  the  procoraco-humeralis  the  procoracoid,  and 
the  supracoracoideus  the  coracoid. 

^  Distal  to  these  come  the  muscles,  the  bellies  of  which  occupy 
the  region  of  the  upper  arm,  and  which  may  thus  form  a  third 
group.  Of  these  there  are  three  that  occupy  the  flexor  aspect, 
and  a  complex  one  with  several  heads  that  lies  upon  the  ex- 
tensor aspect.  Of  the  first  the  two  coraco-brachiales,  longus 
and  brevis,  arise  from  the  coracoid  and  insert  on  the  humerus. 
These  lie  on  the  medial  side.  On  the  outer  or  lateral  side  lies 
the  humero-antebrachialis,  which  arises  along  the  humerus  and 
inserts  by  a  tendon  into  the  proximal  end  of  the  radius.  The 
complex  muscle  on  the  extensor  side,  the  anconeus,  is  con- 
stant in  the  character  of  arising  from  several  heads  and  in 
the  insertion  of  all  by  a  common  tendon  into  the  olecranon 
process  of  the  ulna,  although  the  name  of  "  triceps,"  applied  to 
this  muscle  in  man,  is  objectionable,  since  the  number  of  heads 
is  variable  and  three  is  by  no  means  the  typical  number.  Thus 
here  in  Necturus  there  are  four,  a  central  superficial  one  from 
the  scapula,  a  median  and  a  lateral  one  from  the  humerus  and 
a  median  one  from  the  coracoid.  The  term  anconeus,  bearing 
no  suggestion  of  the  number  of  points  of  origin,  but  referring 
to  its  location  alone  (a^xou,  elbow,  ulna),  is  much  preferable. 


THE    MUSCULAR    SYSTEM  223 

In  mammals,  corresponding  to  the  great  difference  of  struc- 
ture shown  among  the  different  Orders,  there  is  a  great  di- 
versity in  the  appendicular  musculature,  but  if  there  be  taken 
for  comparison  with  the  above  any  of  the  more  primitive 
pentadactylous  quadrupeds,  such  as  a  marsupial,  a  rodent  or  a 
lower  primate,  the  majority  of  the  muscles  in  the  two  can  be 
readily  homologized  (Figs.  59,  60).  Latissimus  dor  si  and 
trapezius  have  greatly  increased  in  extent ;  the  former,  having 
reached  the  mid-dorsal  line,  no  longer  possesses  a  free  dorsal 
margin  anteriorly,  and  posteriorly  shows  no  trace  of  the  primi- 
tive myotomic  slips  of  which  it  was  originally  composed.  A 
slip,  segmented  off  from  its  anterior  edge,  has  become  a  sep- 
arate muscle,  with  the  name  of  teres  major.  The  trapezius 
extends  from  the  occipital  region  of  the  skull,  a  point  which  it 
attains  in  the  higher  salamanders,  along  the  mid-dorsal  line,  to 
a  point  considerably  posterior  to  the  scapula,  where  it  overlaps 
the  latissimus.  It  may  either  be  divided  into  three  distinct 
slips,  anterior,  middle  and  posterior,  or  may  be  in  the  form  of 
an  unbroken  sheet;  and  in  climbing  arboreal  forms,  like  the 
monkeys  and  apes,  and  in  man,  is  of  enormous  extent,  the  two 
covering  the  entire  upper  half  of  the  back  and  prolonged 
posteriorly  into  a  median  point  like  a  monk's  hood,  whence  the 
alternative  name  of  cucullaris,  employed  by  European  anato- 
mists. From  its  anterior  margin  a  bundle  of  fibers  is  set  off 
and  becomes  the  sterno-cleido  mastoideus,  a  muscle  running 
obliquely  across  the  side  of  the  neck  from  the  anterior  end  of 
the  sternum  to  the  skull  just  behind  the  ear,  and  conspicuous 
in  man. 

The  deeper  layer  of  extrinsic  muscles,  levator  scapula  and 
serratns  anterior  [magnus],  have  increased  meanwhile,  prob- 
ably by  the  addition  of  intermediate  slips  that  arise  from  the 
myotomes  between  the  two,  and  become  in  most  mammals  a 
continuous  layer,  the  primary  metamerism  being  expressed  in 
its  slips  of  origin,  which  form  "  digitations,"  or  separate 
pointed  slips  that  arise  from  the  successive  ribs,  or  from  their 
equivalent  processes  in  the  cervical  region.  In  man  these 
muscles  are  again  separated  into  two  by  the  failure  of  certain 


224 


HISTORY   OF   THE    HUMAN    BODY 


FIG.  59.  Diagram  of  human  muscles,  showing  their  relation  to  the 
skeleton.  [After  EUSTACHIUS.] 

This  figure  and  the  next  were  drawn  originally  by  the  gifted  Italian  anatomist, 
Bartolomeo  Eustachio,  who  died  in  1574.  His  very  numerous  anatomical  drawings, 
embracing  all  parts  of  the  body,  were  neglected  for  nearly  two  centuries,  and  were 
finally  collected  and  published,  first,  by  Lancisi  in  1714,  and  later  by  Albinus  in  1761. 
These  figures  are  taken  from  the  latter  edition. 

aa,  attollens  auris;  oc,  occipitalis;  t,  trapezius;  d,  deltoid;  rr,  rhomboideus,  major 
and  minor;  Id,  latissimus  dorsi;  gj,  glutaeus  maximus;  gd,  glutaeus  medius;  gl,  gracilis; 
st,  semitendinosus;  b,  biceps  femoris. 


THE    MUSCULAR    SYSTEM 


225 


Fie.  60.  Diagram  of  human  muscles,  showing  their  relation  to  the 
skeleton.  [After  EUSTACHIUS.  See  explanation  of  Fig.  59.] 

te,  temporalis;  spl,  splenius;  las,  levator  anguli  scapulae;  spa,  serratus  posterior 
superior;  spp,  serratus  posterior  inferior;  sa,  serratus  anterior  (magnus) ;  t,  triceps; 
tm,  teres  major;  gn,  glutaeus  minimus;  p,  piriformis;  st,  semitendinosus,  origin;  bl, 
long  head  of  biceps  femoris,  origin;  bs,  short  head  of  biceps  femoris;  ve,  vastus 
lateralis;  vi,  vastus  medialis;  sm,  semimembranosus;  am,  adductor  magnus;  cr,  vastus 
intermedius;  po,  popliteus. 


226  HISTORY    OF    THE    HUMAN    BODY 

of  the  intermediate  slips.  Aside  from  these  a  second  series  of 
slips,  more  superficial  than  the  above,  appears  in  the  higher 
amphibians,  and  these  develop  in  mammals  into  the  rhomboid- 
ens  system.  This  inserts  into  the  scapula  and  consists  pri- 
marily of  a  slip  from  the  occipital  bone,  rhomboideus  capitis, 
and  one  from  the  vertebral  spines  in  the  interscapular  region, 
rhomboideus  dorsi.  Both  of  these  occur  in  most  mammals, 
but  in  man  rhomboideus  dorsi  alone  is  normally  present,  sub- 
divided into  two  slips,  major  and  minor,,  while  rhomboideus 
capitis  appears  only  as  a  rare  anomaly.  The  pectoralis  in 
many  mammals  forms  a  complex  system  of  distinct  and  semi- 
distinct  portions,  showing  at  least  a  superficial  and  a  deep 
layer.  In  man  and  the  anthropoids  these  two  layers  are  repre- 
sented by  two  muscles,  pectoralis  major  and  minor  respectively. 
The  subclavius  is  a  differentiation  from  the  deeper  layer. 

Of  the  intrinsic  group  the  dorsalis  scapula?  becomes  mainly 
the  deltoid,  often  divided  into  several  portions,  spino-deltoid, 
acromio-deltoid,  etc.,  but  single  in  man.  A  small  portion  of 
this  muscle  becomes  the  teres  minor,  topographically  associated 
with  the  teres  major,  derived  from  the  latissimus.  VThe  sufera- 
coracoideus  is  probably  represented  by  the  supra-  and  ivrfra- 
spinati,  which  have  extended  dorsally  over  the  scapula,  pushing 
their  way  beneath  the  deltoid,  as  this  muscle  has  gradually 
lifted  itself  up  from  the  general  outer  surface  of  that  bone. 
The  procoraco-humeralis  seems  to  have  become  lost,  together 
with  the  axial  slips  that  insert  into  the  procoracoid. 

The  anconeus,  the  extensor  muscle  of  the  upper  arm,  varies 
mainly  in  the  number  and  position  of  its  heads,  and  not  in  its 
insertion  or  general  position.  Its  identity  with  the  human 
triceps  has  been  already  commented  on.  On  the  flexor  side  of 
the  upper  arm  the  history  is  not  as  plain.  In  the  mammals 
there  are  two  long  muscles  that  insert  into  the  forearm,  the 
biceps  brachii,  that  arises  from  the  shoulder  girdle  and  inserts 
by  a  tendon  into  the  proximal  portion  of  the  radius,  and  the 
brachialis  [anticus]  that  arises  along  the  shaft  of  the  humerus, 
and  inserts  into  the  proximal  end  of  the  ulna.  Aside  from 
these,  there  is  a  coraco  brachialis,  from  the  coracoid  process 


THE    MUSCULAR   SYSTEM  227 

to  the  shaft  of  the  humerus.  These  do  not  homologize  readily 
with  the  muscles  of  the  same  region  in  urodeles,  but  the  last 
muscle,  which  appears  in  some  mammals  as  two,  compares  well 
with  those  of  like  name  in  Necturus.  This  leaves  the  humero- 
antebrachialis  to  be  compared  with  both  biceps  and  brachialis, 
and  it  may  well  be  the  ancestral  form  from  which  both  have 
originated.  In  origin  it  is  like  the  latter,  and  shows  no  sim- 
ilarity writh  the  biceps,  which  arises,  usually  by  a  single  head, 
from  the  scapula  and  the  coracoid  process ;  it  is,  however,  pre- 
cisely like  the  biceps  in  its  mode  of  insertion,  and  must  be  at 
least  in  part  homologous  with  this  latter  muscle. 

The  second  region  to  be  considered  is  the  proximal  portion 
of  the  posterior  limb  and  includes  the  muscles  of  the  pelvic 
girdle  and  thigh.  As  in  the  skeleton  there  is  in  the  muscula- 
ture little  suggestion  of  serial  homology  between  the  two  pairs 
of  appendages,  although  in  Necturus  the  two  limbs  closely  cor- 
respond in  the  distal  portion.  A  fundamental  difference  in  the 
muscles  of  the  two  girdles  is  that  in  the  posterior  limb  they 
are  nearly  all  intrinsic,  and  arise  from  the  appendicular  skel- 
eton, while  in  the  anterior  limb  an  extensive  system  of  extrinsic 
muscles  controls  in  part  both  the  girdle  as  a  whole  and  the 
proximal  part  of  the  free  limb.  This  difference  is  undoubtedly 
correlated  with  the  definite  attachment  of  the  posterior  girdle 
to  the  axial  skeleton  through  the  formation  of  a  sacrum,  while 
in  the  anterior  girdle  there  is  either  no  attachment  to  the  verte- 
bral column,  or  a  freely  movable  one  through  clavicle, 
sternum  and  ribs. 

In  Necturus  (Fig.  61),  in  which  the  pelvic  girdle  is  in  the 
form  of  a  flat  pubo-ischiadic  plate  and  a  narrow  ilium,  the 
muscles  are  naturally  divided  into  those  of  the  outer  (ventral), 
and  those  of  the  inner  (dorsal)  side  of  the  plate,  and,  thirdly, 
those  which  arise  from  the  ilium.  Of  the  first  there  are  two, 
forming  as  many  layers  on  the  outer  side  of  the  plate;  the 
pubo-ischio-tibialis,  which  runs  down  the  inner  side  of  the  leg, 
passes  the  femur  and  inserts  into  the  proximal  end  of  the  tibia, 
and  the  pubo-ischio-femoralis  externus,  which  inserts  into  the 
femur.  Upon  the  inner  side  of  the  plate  there  is  a  single  large 


228 


HISTORY   OF   THE    HUMAN    BODY 


muscle,  the  pub o-ischio-femor alls  internus,  the  fibers  of  which 
part  to  accommodate  the  ilium,  but  reunite  again  upon  the 
outer  side  of  this  obstacle.  This  also  inserts  into  the  femur. 
Aside  from  these,  a  narrow  band,  the  pubo-tibialis,  arises  from 
the  lateral  edge  of  the  pubo-ischiadic  plate,  and  inserts  in  the 
tibia.  From  the  ilium  arise  an  ilio-extensorius,  which  inserts 
into  the  femur,  an  ilio-femoralis,  also  to  the  femur,  and 


FIG.  61.     Ventral  view  of  the  pelvic  muscles  of  Necturus. 

pife,  pubo-ischio-femoralis  externus;  pifi,  pubo-ischio-femoralis  internus;  pit,  pubo- 
ischio-tibialis;  pt,  pubo-tibialis;  isf,  ischio-femoralis;  isc,  ischio-caudahs;  cpit,  caudali- 
pubo-ischio-tibialis;  cf,  caudali-fermoralis;  ra,  rectus  abdominus.  Other  designations: 
gl,  cl.,  cloacal  gland;  Pub.,  pubic  portion  of  pubo-ischiadic  plate;  isch.,  ischiadic 
portion  of  the  same. 

an  ilio-nbularis,  a  narrow  band,  which  inserts  into  the  proximal 
end  of  the  fibula.  A  femoro-fibularis,  also  band-like,  arises 
from  the  flexor  side,  of  the  femur,  and  inserts  into  the  fibula, 
near  the  last.  The  only  extrinsic  muscles  are  found  in  a  set 
of  three  caudal  muscles,  which  arise  along  the  sides  of  the  tail 
not  far  below  the  pelvis,  and  run  in  a  sheath  anteriorly  to  the 


THE    MUSCULAR    SYSTEM  229 

posterior  limbs,  inserting  the  one  into  the  ischium,  another 
into  the  femur,  and  the  third,  into  the  margin  of  the  pubo- 
ischio-tibialis. 

Unlike  the  anterior  limb,  in  which  the  most  of  the  muscles 
occurring  in  Necturus  may  be  readily  recognized  in  mammals, 
the  homologies  of  the  proximal  portion  of  the  pelvic  limb  are 
all  more  or  less  doubtful.  As  a  beginning,  there  are  found 
upon  the  outer  side  of  the  pubo-ischium  in  mammals  the 
obturator  externus,  the  adductor es,  the  gracilis,  which -belongs 
with  the  adductor  group,  and  possibly  the  sartorius.  Of  these 
the  obturator  is  probably  the  homologue  of  the  pubo-ischio- 
femoralis  externus,  and  the  remainder  may  be  derivatives  of 
the  pubo-ischio-tibialis,  in  spite  of  the  difference  in  respect  to 
insertion.  The  pubo-ischio-femoralis  internus  seems  to  give 
rise  to  the  obturator  internus -with  the  two  associated  gemelli, 
as  well  as  to  the  ilio-psoas  complex,  which  appears  first  as  a 
distinct  muscle  in  reptiles.  The  ilio-extensorius  is  probably  the 
prototype  of  the  great  complex  of  the  front  of  the  thigh,  quad- 
riceps femorisf  composed  of  the  three  vasti,  externus,  medialis 
\_crur  eus~\  and  internus,  and  the  rectus  femoris.  The  glutai 
are  probably  derived  from  the  ilio-femoralis. 

Of  the  muscles  of  the  posterior  aspect  of  the  thigh,  enor- 
mously developed  in  mammals,  the  inner  ones,  Mm.  semimem- 
branosus  and  semitendinosus,  are  probably  also  derived  from 
the  pubo-ischio-tibialis,  while  the  two  heads  of  the  outer  one, 
the  biceps  femoris,  come  from  two  distinct  sources,  and  in 
many  mammals  are  separate  muscles.  The  derivation  of  the 
long  head  is  uncertain,  but  it  may  be  homologous  with  the  ilio- 
•fibularis  of  urodeles,  in  spite  of  the  difference  in  origin.  The 
short  head,  on  the  other  hand,  is  derived  from  the  glutaeal 
group,  and  is  identical  with  the  long  narrow  band,  described 
in  many  mammals  as  the  teniiissimus.  Since  it  is  associated 
with  the  long  head  to  form  a  "  biceps  "  muscle  in  a  few  mam- 
mals only,  including  man  and  several  apes,  this  slip  is  best 
considered  as  a  separate  muscle  of  the  glutaeal  group,  under 
the  name  of  glutceo-cruralis.  The  caudal  group  of  muscles, 


230  HISTORY    OF   THE    HUMAN    BODY 

which  extends  in  urodeles  from  the  sides  of  the  tail  to  the 
ischium  and  femur,  persists  with  some  modifications  in  mam- 
mals ;  in  man  and  the  higher  anthropoids,  in  which  the  reduc- 
tion of  the  caudal  vertebrae  restricts  the  origin,  the  group  is 
represented  by  a  single  muscle,  the  piriformis,  extending  from 
the  coccygeal  region  across  to  the  femur. 

The  muscles  of  the  distal  portion  of  the  vertebrate  chirop- 
terygium,  that  is,  from  elbow  or  knee  on,Jaside  from  the  mod- 
ifications imposed  upon  them  by  the  varying  shapes  of  the 
limbs  themselves,  and  the  great  difference  in  their  use,  are,  in 
their  essential  features,  quite  similar  in  all  living  forms;  and 
in  their  differences  show  the  modifications  of  a  primary  type 
due  to  environment  rather  than  the  suggestions  of  an  historic 
development  of  that  type.  The  study  is,  therefore,  one  mainly 
of  the  adaptations  of  a  given  set  of  elements,  rather  than  a 
phylogenetic  history,  which  latter,  as  is  the  case  also  with  the 
bones  of  the  same  region,  must  be  sought  in  the  gap  separating 
fin  and  hand,  that  is,  in  the  phylogenetic  stages  represented 
by  lost  forms  of  ganoids,  stegocephali,  and  their  allies.  The 
salamander  Necturus,  probably  the  nearest  approach  to  this 
series  represented  by  living  fauna,  offers  in  its  distal  muscles 
some  few  suggestions  of  an  earlier  phylogenetic  stage,  and  is 
thus  of  fundamental  importance  in  the  present  inquiry.  The 
well-nigh  complete  correspondence  in  the  fore  and  hind  limb 
as  regards  not  only  bones  and  muscles,  but  other  parts  as  well, 
has  been  commented  on  above  and  offers  strong  support  for 
the  doctrine  of  serial  homology,  to  be  considered  later.  There 
are,  also,  as  is  the  case  with  higher  forms,  some  traces  of  a 
correspondence  between  the  dorsal  and  ventral  surfaces  of  a 
single  paw,  giving  a  suggestion  of  the  derivation  of  the 
chiridial  musculature  from  a  fin-like  precursor,  in  which  the 
jointed  rays  (digits)  were  supplied  by  similar  muscular  ele- 
ments applied  both  dorsally  and  ventrally,  as  in  present-day 
fishes.  The  following  description  is  that  of  the  anterior  limb, 
but  with  the  substitution  of  the  terms  tibia  and  -fibula  for  radius 
and  ulna,  tarsus  for  carpus,  and  so  on,  it  will  be  found  almost 
equally  applicable  to  the  posterior  one.  In  a  few  cases  a  muscle 


THE    MUSCULAR    SYSTEM  231 

which  is  well  developed  in  the  anterior  limb  is  small  or  want- 
ing in  the  posterior,  and  thus  the  former  is  a  little  more 
typical* 

The  dorsal  aspect  of  the  antebrachium  (Fig.  62,  a  and  b)  is 
largely  taken  up  superficially  by  a  single  muscular  mass,  M. 
dgrsalis  antebrachii  (da.)  which  arises  from  the  distal  end  of 
the  humerus.  This  separates,  spreads  out  over  the  ante- 
brachium, and  divides  distally  into  four  slips,  three  for  the  in- 
ter-digital spaces  and  one  for  the  ulnar  side  of  digit  V.  Each 
of  these  in  turn  divides  into  two,  which  insert  by  tendons  into 
the  bases  of  the  adjoining  metacarpals.  The  muscle  is  thus  an 
abductor-adductor  complex,  furnishing  the  digits  with  lateral 
motions,  but  without  any  power  in  extending  them.  The 
radial  aspect  of  digit  II  is  alone  unsupplied  from  this  system, 
and  this  deficiency  is  made  up  by  the  supinator  (s),  a  muscle 
which  underlies  the  former,  arising  from  the  ulnar  side  of  the 
carpus.  It  crosses  the  limb  obliquely,  and  inserts  into  the 
internal  or  free  aspect  of  metacarpal  II.  Extension  of  the 
digits  is  effected  by  four  short  muscles,  Mm.  extensores  breves 
(x,  x) ,  \vhich  arise  from  the  distal  row  of  carpalia  and  become 
continued  into  tendons  that  lie  along  the  dorsum  of  the  sep- 
arate digits  and  insert  into  the  bases  of  the  terminal  phalanges. 
Partly  along  the  sides  of  the  dorsalis,  and  partly  covered  by  it, 
thus  forming  a  deeper  layer,  are  two  long  muscles,  associated 
respectively  with  radius  and  ulna,  Mm.  extensor  radialis  and 
extensor  ulnaris  (er.  and  eu.).  These  arise  from  the  humerus 
with  the  dorsalis  and  insert,  the  one  along  the  shaft  of  the 

*  In  one  point  the  free  limb  of  Necturus  diverges  from  what  is  gener- 
ally believed  to  be  the  typical  chiropterygium,  and  that  is,  it  possesses  but 
four  digits  in  each  extremity  instead  of  the  canonical  five  which  is  usually 
considered  primitive.  Since  the  nearest  ally  of  this  species,  the  cave 
form,  Proteus,  exhibits  a  still  greater  reduction  of  digits  (anterior,  3; 
posterior,  2),  it  has  been  presumed  that  this  is  in  both  cases  a  secondary 
reduction.  Certain  facts,  however,  lead  one  to  think  that  the  first  land 
vertebrates  possessed  a  smaller  number  of  digits  than  five,  and  if  this  be 
so,  the  condition  in  these  two  salamanders  is  primitive,  and  not  a  second- 
ary reduction.  According  to  the  reduction  theory  digit  I  is  assumed  to  be 
the  one  lost,  and  in  accordance  with  this  the  four  digits  present  are 
designated  here,  both  in  text  and  illustrations,  as  II-V. 


232 


HISTORY    OF    THE    HUMAN    BODY 


radius  and  into  certain  of  the  radial  carpals,  the  other  along 
the  ulna  and  into  ulnar  carpals. 

The  ventral  (palmar)  aspect  .of  the  limb  is  more  complicated 
in  respect  to  its  muscles.  These  are  covered  superficially  by  a 
dense  palmar  fascia  or  aponeurosis  (//>),  *°  which  many  of  the 


IT 


III 


FIG.  62.    Muscles  of  the  fore-paw  of  Necturus ;  dorsal  aspect 

(a)    Superficial  muscles.      (b)    Deeper  muscles. 

da,  djrsalis  antebrachii,  its  separate  insertions  into  the  metacarpalia  are  shown 
in  (b) ;  er,  radial  extensor;  eu,  ulnar  extensor;  fit,  ulnar  flexor,  showing  from  the 
other  side;  xx,  extensores  breves;  s,  supinator;  22,  intermetacarpales. 

ventral  muscles  are  attached.  This  aponeurosis  is  a  continu- 
ation of  the  fascia  covering  the  ventral  muscles  of  the  forearm 
and  appears  at  its  thickest  and  densest  as  it  passes  over  the  car- 
pal and  metacarpal  regions.  At  the  separation  of  the  digits  this 
aponeurosis  is  also  divided  into  four  bands  which  run  along 
the  ventral  surface  of  the  separate  digits  and  insert  into  the 


THE    MUSCULAR    SYSTEM 


233 


terminal  phalanges.  These  slips  are  functionally  and  probably 
morphologically  the  long  flexor  tendons  (ft.),  and  correspond 
in  a  way  to  the  long  extensor  tendons  of  the  dorsal  side,  but  it 
must  be  remembered  that  here  they  are  the  continuation,  not 
of  muscular  bellies,  but  of  a  non-contractile  aponeurosis.  The 
entire  aponeurosis,  however,  is  caused  to  move  by  serving  as 
point  of  insertion  of  several  muscles,  more  proximally  placed, 


III 


III 


FIG.  63.     Muscles  of  the  fore-paw  of  Necturus;  ventral  aspect. 

(a)    Superficial   muscles,      (b)    Deeper   muscles. 

fp,  palmar  fascia;  ps,  palmaris  superficialis ;  pp,  palmaris  profundus;  uc,  ulno- 
carpalis;  fu,  ulnar  flexor;  fr,  radial  flexor;  pr,  pronator;  yy,  flexores  breves  super- 
ficiales;  tt,  terminal  tendons  of  the  palmar  fascia. 

the  palmaris  superficialis  (ps) ,  which  inserts  along  its  proximal 
edge,  or,  more  exactly,  between  its  two  layers,  which  thus 
invest  the  muscle,  and  the  palmaris  profundus  (pp),  which  is 
entirely  covered  by  the  aponeurosis  and  inserts  into  its  dorsal 
(internal)  side.  The  action  of  these  muscles  upon  the  aponeu- 
rosis causes  it  to  act  indirectly,  through  its  digital  slips,  upon 
the  separate  digits,  and  cause  a  complete  flexion.  Aside  from 
the  palmaris  system,  the  physiological  action  of  which  is  that 


234  HISTORY   OF   THE   HUMAN   BODY 

of  a  system  of  flexors,  there  are  two  sets  of  short  flexors,  Mm. 
flex  ores  breves  super  ficiales  and  Hex  ores  breves  profundi,  each 
consisting  of  four  muscles,  one  to  each  digit.  The  super- 
ficiales arise  from  the  distal  row  of  carpalia,  and  pass  into  ten- 
dons, which,  encountering  the  long  slips  of  the  aponeurosis, 
divide  into  two  lateral  tendons  and  insert  upon  the  sides  of  the 
penultimate  phalanges.  The  Hexores  profundi  lie  close  to  the 
bone,  arise  beneath  the  former,  and  insert  into  the  bases  of  the 
proximal  phalanges.  There  are  here  also,  as  on  the  dorsal 
side,  two  long  muscles  which  arise  from  the  humerus  and  insert 
along  the  shafts  of  radius  and  ulna  and  into  the  corresponding 
sides  of  the  carpals,  serving  as  flexors  of  antebrachium  and 
manus  as  a  whole.  These  are  respectively  the  flexor  radlalis 
.and  flexor  ulnaris  (fr  and  fu). 

The  ventral  muscles  thus  far  enumerated,  act  either  directly 
T>r  indirectly  as  flexors,  but  beneath  all  of  these  is  a  set  of  short 
abductors  and  adductors  of  the  metacarpals,  abductores  and 
adductores  breves,  which  correspond  in  function  to  the  large 
muscle  mass  of  the  dorsal  aspect,  M.  dorsalis  antebrachii,  with 
its  abductor  and  adductor  tendons.  These  extend  across  the 
interval  between  the  distal  carpalia  and  the  metacarpals,  and 
like  those  of  the  dorsal  mass,  supply  both  sides  of  the  two  inner 
digits,  III  and  IV,  and  the  inner  sides  of  II  and  V.  As  in 
the  case  of  the  abductor  and  adductor  system  of  the  dorsal  side, 
M.  dorsalis  antebrachii,  the  internal  (radial)  side  of  digit  II 
remains  unsupplied  from  this  system  and  the  deficiency  is  made 
good  by  the  pronator  (pr),  a  muscle  which  lies  obliquely  across 
the  antebrachium  and  is  related  to  the  skeletal  parts  precisely 
as  is  the  supinator  of  the  dorsal  side.  Like  the  latter  it  arises 
from  the  shaft  of  the  ulna  and  passes  obliquely  downwards  to 
the  radial  side  of  the  limb,  where  it  inserts  by  a  tendon  into  the 
radial  side  of  the  base  of  metacarpal  II. 

Deepest  of  all,  beneath  the  short  abductors  and  adductors, 
and  reached  equally  well  from  either  dorsal  or  ventral  aspect, 
a  set  of  three  intermetacarpales  stretch  their  fibers  across  the 
interspaces  between  the  separate  metacarpals  and  act  either  as 


THE    MUSCULAR    SYSTEM  235 

abductors  or  adductors  of  the  separate  digits   (Fig.   62,  b, 

«~<~  \ 

Reviewing  the  conditions  in  this,  probably  the  most  primi- 
tive chiropterygium  now  left  to  us,  several  interesting  points 
become  manifest.  The  digits  are  moved  in  two  ways,  either 
flexed  and  extended  or  moved  sideways,  but  while  the  system 
which  provides  for  this  latter  form  of  motion  is  extremely  well 
perfected,  that  for  flexion  and  extension  is  not.  For  abduc- 
tion and  adduction  there  are  typically  five  separate  muscles  for 
each  digit,  that  is,  two  ventral,  two  dorsal  and  one  intermeta- 
carpal,  while  for  flexion  and  extension,  aside  from  the  system 
supplied  by  an  aponeurosis,  and  evidently  a  newly  introduced 
feature,  there  are  but  three.  This  extreme  perfection  of  the 
sideways  movement  of  the  digits  in  the  most  primitive  chirid- 
ium  knowm,  together  with  the  weak  and  makeshift  arrange- 
ments for  bending  and  straightening  the  digits,  strongly  sug- 
gest the  derivation  of  the  chiridial  type  from  one  in  which  the 
digits  (fin-rays?)  required  to  be  constantly  opened  and  shut 
by  lateral  movements,  precisely  as  in  the  case  of  the  fins  of  most 
fishes. 

During  later  phylogenetic  history  there  is  an  evident  tend- 
ency to  increase  the  efficiency  of  the  flexor-extensor  system  and 
diminish  that  of  the  abductors  and  adductors,  except  in  the  case 
of  the  two  digits  that  form  the  ends  of  the  series  (I  and  V), 
and  the  most  of  these  changes  have  already  occurred  among 
the  higher  urodeles.  Thus  in  Cryptobranchus  the  dorsalis 
antebrachii,  which  in  Necturus  serves  as  an  abductor-adductor 
system  and  terminates  at  the  base  of  the  metacarpals,  is  con- 
tinued into  four  long  tendons,  which  insert  into  the  terminal 
phalanges,  and  thus  becomes  the  extensor  communis  digitorum, 
although  in  the  hind  limb  at  least,  from  the  sides  of  the  long 
tendons,  small  lateral  slips  extend  still  to  the  sides  of  the 
metacarpals,  the  remains  of  the  abductor-adductor  system. 
The  short  extensors  become  more  complicated  than  in  Nec- 
turus, but  insert  along  the  proximal  part  of  the  digits  and 
are  no  longer  continued  into  long  tendons  to  the  ter- 


236  HISTORY   OF    THE    HUMAN    BODY 

minal  phalanges,  as  that  function  has  been  usurped  by  the 
other  muscle. 

Upon  the  ventral  side  a  more  intimate  connection  between 
the  tendons  of  the  palmar  aponeurosis  and  the  associated 
muscles  gives  rise  to  the  system  of  long  flexors,  as  found  in  the 
higher  vertebrates.  In  the  arm,  where  this  history  has  been 
most  completely  followed,  the  palmaris  muscles,  superficial  and 
deep,  uniting  with  the  palmar  fascia  and  its  long  tendons,  form 
the  two  long  flexors  characteristic  of  mammals,  flexor  digit- 
orum  sublimis  and  profundus.  In  the  monotremes  the  bellies 
form  a  common  mass,  flexor  communis  digitoruni,  although 
with  double  tendons  to  the  separate  digits,  a  deep  tendon  which 
inserts  on  the  terminal  phalanx  and  a  superficial  tendon  which 
forks.  The  two  resulting  parts  insert  upon  the  edges  of  the 
penultimate  phalanx,  and  allow  the  deep  tendons  to  pass 
through  between  them.  A  later  differentiation  of  the  belly 
divides  it  into  a  flexor  profundus,  continued  into  the  deep  ten- 
don, and  a  flexor  sublimis,  continued  into  the  superficial  tendon. 
The  most  superficial  of  the  fibers  separate  into  the  somewhat 
inconstant  "  palmaris  longus"  The  flexor  pollicis  longus  of 
man  belongs  with  the  profundus. 

The  tendons  of  the  short  superficial  flexors  of  amphibians 
become  mainly  employed  in  the  formation  of  the  profundus 
tendons,  while  the  bellies  degenerate,  but  those  associated  with 
digits  I  and  V  develop  in  the  mammalian  hand  and  foot  into 
special  muscles  connected  with  those  digits,  such  as  the  abduc- 
tors of  pollex  and  minimus,  the  opponentes  of  the  same,  and 
the  flexor  brevis  minimi  digiti.  The  short,  deep  flexors  of  the 
amphibians,  flex  ores  breves  profundi,  become  the  mammalian 
lumbricales;  and  the  still  deeper  set  of  abductors  and  adduc- 
tors, together  with  the  intermetacarpales,  become  the  two  sets 
of  interossei,  palmares  and  dorsales,  the  latter  arising  upon  the 
ventral  aspect  and  coming  through  to  the  dorsal  side  during 
development. 

The  four  muscles  which  in  Necturus  lie  along  the  ulnar  and 
radial  sides  of  the  antebrachium  on  both  dorsal  and  palmar 


THE    MUSCULAR    SYSTEM  237 

aspect,  and  furnish  a  ftexor  and  an  extensor  for  each  side,  are 
continued  with  some  modifications  in  higher  animals.  The 
origin  from  the  distal  end  of  the  humerus  remains  the  same, 
but  the  insertions  along  the  shafts  of  ulna  and  radius  are  given 
up,  and  are  either  confined  to  the  carpal  bones  of  the  corre- 
sponding sides  or  a  new  tendinous  insertion  is  acquired  which 
extends  to  the  base  of  some  metacafpal,  the  muscles  becoming 
•flexor  carpi  radialis,  extensor  carpi  ulnaris,  and  so  on.  The 
extensor  carpi  radialis  of  mammals  becomes  divided  into  two 
similar  muscles,  longus  and  brevis,  which  insert  into  the  bases 
of  metacarpals  II  and  III  respectively. 

A  final  group  of  limb  muscles  are  the  pronator  and  supinator f 
which  give  the  limb  the  power  of  turning  about  its  axis,  thus 
crossing  or  tending  to  cross  the  two  bones  of  the  forearm  or 
lower  leg.  Of  these  the  pronator  lies  upon  the  flexor,  the 
supinator  upon  the  extensor  aspect  of  the  limb,  their  fibers 
extending  diagonally  across  from  one  bone  to  the  other.  In 
Necturus  there  is  one  upon  each  aspect,  the  character  of  which 
suggests  their  derivation  from  the  primary  system  of  abductors 
and  adductors. 

The  striking  correspondence  in  many  features  between  the 
anterior  and  posterior  limb,  especially  shown  in  cases  in  which 
the  two  are  used  in  a  similar  way,  has  naturally  led  to  the 
theory  of  the  serial  homology  between  them,  that  is,  an  original 
homology,  not  between  different  animals,  as  is  usually  meant 
by  the  term,  but  between  different  parts  of  the  same  animal. 
The  theory  presupposes  a  time  at  which  both  sets  of  limbs  were 
exactly  alike,  part  for  part,  and  thus  the  final  results,  however 
unlike  in  the  two  cases,  are  referable  to  a  single  ground  plan 
from  which  both  have  been  derived.  It  would  be  thus  possible 
to  homologize  bone  for  bone,  muscle  for  muscle,  and  to  extend 
the  parallelism  to  the  vessels  and  nerves  as  well. 
•  A  strong  proof  of  this  is  afforded  by  the  close  similarity  of 
the  two  limbs  in  the  lowest  of  the  amphibians,  as  stated  above, 
for  here,  as  shown  especially  in  that  form  which  is  probably 
the  most  primitive  of  all,  the  correspondence  is  very  remark- 


238  HISTORY   OF   THE    HUMAN    BODY 

able.  It  must  be  noted,  however,  that  this  serial  homology  is 
clear  only  in  the  case  of  the  distal  portion  of  the  limb,  the  part 
beyond  the  elbow  or  knee,  while  in  the  portion  proximal  to  this 
point,  there  is  very  little  suggestion  of  such  a  parallelism. 
From  this  may  be  drawn  the  following  conclusions :  Granting 
that  both  limbs  have  arisen  from  a  similar  origin,  and  were 
alike  at  the  start,  it  is  allowable  to  suppose  that  the  distal 
portions,  being  used  in  a  similar  manner,  have  either  retained 
their  primitive  structure,  or  have  differentiated  alike,  up  to  the 
point  exhibited  by  the  present-day  urodeles ;  the  proximal  por- 
tions, facing  from  the  start  radically  different  problems  con- 
nected with  the  poise  of  the  body,  the  varied  action  of  different 
parts  of  the  trunk,  and  other  differentiating  factors,  have 
become  modified  along  different  lines,  and  have  attained  results 
that  suggest  little  of  the  original  homology. 

The  fin-fold  theory  of  the  origin  of  the  limbs,  given  in 
the  previous  chapter,  throws  but  little  light  upon  the  theory  of 
serial  homology,  and,  it  must  be  confessed,  even  stands  some- 
what in  the  way  of  such  an  hypothesis,  since,  although  in  its 
primitive  form,  the  fin-fold  may  be  considered  to  have  been 
made  up  of  similar  elements,  repeating  themselves  metameric- 
ally,  and  appearing  probably  as  skeletal  rays  supplied  with 
muscles  from  the  trunk  musculature  in  the  form  of  "  myotomic 
buds/'  yet  there  is  no  suggestion  that  an  identical  number  of 
these  elements  was  originally  taken  in  the  case  of  the  two  sets 
of  limbs  or  that  the  strictly  pentadactylous  character  of  the 
hand  form  could  have  been  in  any  sense  primitive,  or  could 
have  existed  at  the  time  at  which  the  two  limbs  might  be  sup- 
posed to  have  been  strictly  identical. 

However,  as  opposed  to  all  theory  in  the  matter,  and  it  must 
be  remembered  that  the  fin-fold  theory  itself  rests  upon  very 
little  actual  evidence,  there  is  the  fact  of  the  actual  close  corre- 
spondence in  the  fore  and  hind  limbs  of  urodeles  in  general, 
and  especially  in  the  case  of  Necturus,  the  particular  form 
which  from  other  reasons  is  considered  especially  primitive, 
perhaps  even  the  oldest  living  representative  of  all  animals 
possessing  the  hand  form  of  appendage. 

Among  mammals  the  limbs  of  the  primates,  even  in  the 


THE    MUSCULAR   SYSTEM  239 

slightly  modified  form  possessed  by  bipedal  man,  are  quite 
primitive,  and,  together  with  those  of  the  allied  insectivores 
and  rodents,  retain  the  typical  five  digits,  a  character  in  which 
most  of  the  Carnivora  and  nearly  all  of  the  ungulate  groups 
show  a  much  greater  specialization.  Thus  the  distance  sep- 
arating primates  from  the  amphibians  is  not  very  great,  and  it 
is  therefore  not  surprising  that  in  the  distal  portion  an  homol- 
ogy,  not  merely  of  the  bones,  but  of  the  muscles  as  well,  is  still 
quite  evident.  In  the  chapter  on  the  endoskeleton  the  close 
correspondence  was  pointed  out  between  the  bones  of  the  arm 
and  hand  and  those  of  the  leg  and  foot;  here  the  similar  corre- 
spondence may  be  considered  between  the  muscles  of  these 
parts,  the  subject  being  confined  in  this  case,  however,  to  the 
distal  portion,  that  is,  the  portion  from  the  elbow  or  knee  on. 

Take  in  the  first  place  the  set  of  four  radial  and  ulnar 
muscles,  which  in  their  final  form  in  the  primates  become  five, 
namely,  the  flexor  carpi  ulnaris,  flexor  carpi  radialis,  extensor 
carpi  ulnaris  and  the  two  extcnsores  carpi  radiales,  longus  and 
brevis. 

Their  homologues  in  the  leg  have  naturally  become  modified 
through  the  difference  in  function  and  the  formation  of  a  heel, 
and  their  determination  is  perhaps  the  least  clear  of  any  of  the 
distal  limb  muscles.  There  are,  however,  to  correspond  to  the 
two  flexors,  the  tibialis  posterior  and  the  soleus-gastrochnemius 
complex,  of  which  the  first,  a  tibial  flexor,  represents  the  radial 
one,  and  the  second,  primarily  a  fibular  flexor,  the  correspond- 
ing muscle  on  the  ulnar  side.  Upon  the  extensor  aspect,  the 
tibalis  anterior  may  be  the  serial  homologue  of  both  radial 
extensors,  while  the  ulnar  extensor  is  represented  by  the  two 
peroncci,  longus  and  brevis.  In  tabular  form  the  above  homol- 
ogies  appear  as  follows : 

ARM  .       LEG 

Flexor  carpi  radialis Tibialis  posterior 

Flexor  carpi  ulnaris Soleus-gastrochnemius 

Extensor  carpi  radialis    longus  ) 

Extensor  carpi  radialis    brevis    J TtbiallS  anU™r 


Extensor  carpi  ulnaris Peronaus  \ 


longus 
brevis 


240  HISTORY    OF    THE    HUMAN    BODY 

The  set  of  supinators  and  pronators  may  next  be  considered, 
and  in  Necturus,  in  which  in  each  limb  there  is  a  single 
supinator  on  the  extensor  side,  and  a  single  pronator  on  the 
flexor  side,  the  homologies  are  evident.  In  all  cases  they 
extend  from  a  more  proximal  origin  upon  the  outer  side  (ulnar 
or  fibular)  to  a  more  dorsal  insertion  upon  the  inner  (radial  or 
tibial).  In  the  human  arm  there  are  two  pronators,  teres  and 
quadratus,  but  as  these  are  continuous  in  many  marsupials  and 
carnivores,  they  may  be  considered  as  derivatives  of  the  single 
urodele  muscle.  Their  homologue  in  the  leg  is  undoubtedly 
the  popliteus.  Upon  the  dorsal  side  most  works  on  human 
anatomy  record  two  supinators,,  longus  and  brevis,  but  as  the 
longus  [=brachioradialis  BNA]  is  really  a  portion  of  M. 
brachialis,  and  belongs  with  the  upper  arm,  the  only  true 
supinator  is  the  one  designated  brevis  [—supinator,  BNA], 
undoubtedly  the  same  as  that  in  the  urodeles.  Its  homologue 
in  the  leg  seems  to  have  disappeared. 

The  remaining  muscles,  those  controlling  the  action  of  the 
separate  digits,  are  still  more  in  accord,  and  that  too  in  spite  of 
the  great  difference  in  use  between  the  hand  and  foot,  especially 
in  civilized  man,  suggesting  the  conservatism  of  these  parts, 
and  the  fact  that  it  is  easier  to  keep  a  complicated  structure, 
when  once  obtained,  even  when  not  used  in  all  its  parts,  than 
to  replace  it  with  a  simple  structure  without  unnecessary  parts, 
provided  only  that  the  more  complicated  structure  is  in  no  case 
detrimental  to  the  effective  working  of  the  organ  in  its  simpli- 
fied function.  It  is  often  presumed  that  the  reason  why  such 
changes  as  these  have  not  occurred  is  that  the  time  has  been 
insufficient  to  effect  it,  but  this  is  not  the  case.  The  only 
reason  for  an  adaptation  lies,  not  in  the  lapse  of  time,  which 
in  itself  is  powerless  to  effect  even  the  slightest  change,  but  in 
the  question  of  expediency  for  the  animal,  that  is,  whether  the 
part  comes  within  the  power  of  natural  selection  or  not.  In 
the  present  case  the  foot  of  man  and  his  immediate  ancestors 
has  borne  its  present  shape  from  pre-glacial  times,  a  period 
given  by  conservative  estimates  at  50,000  to  100,000  years,  and 
has  as  yet  undergone  but  little  change  along  the  line  of  reduc- 


THE    MUSCULAR    SYSTEM 


241 


tion.  A  needful  progressive  change  has  indeed  taken  place, 
namely,  the  development  of  the  peroncuits  tertius,  a  muscle  for 
lifting  the  outer  edge  of  the  foot  and  thus  counteracting  the 


1    1 

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o 

S 


p  « 

^ 

d 


tendency  to  walk  exclusively  upon  this  portion ;  but  the  change 
in  the  line  of  reduction  of  a  needless  complexity  of  parts  has 


242  HISTORY    OF   THE    HUMAN    BODY 

been  but  slight,  simply  because  there  has  been  no  need  of  such 
a  modification. 

To  review  the  complete  homology  of  the  muscles  of  the 
hand  and  foot  would  prove  too  long  a  task  for  the  present 
work,  but  a  large  part  of  the  correspondence  may  be  presented 
in  the  form  of  a  diagram  (Fig.  64),  which  gives  all  the  muscles 
of  the  flexor  surface,  excepting  the  lumbricales  and  the  intcr- 
ossei.  In  both  there  is  seen  a  double  system  of  flexor  tendons, 
perforantes  and  perforati;  the  first  digit  has  the  perforans 
alone,  in  both  hand  and  foot,  and  in  the  foot  a  perforatus  is 
wanting  in  digit  V,  possibly  a  regressive  change.  In  the 
anterior  limbs  both  of  these  systems  are  long  muscles,  their 
bellies  lying  along  the  forearm,  while  in  the  foot  the  belly  be- 
longing to  the  three  perforated  tendons  of  digits  II  to  IV 
is  a  short  one,  confined  to  the  foot  region.  The  two  outer 
digits  are  richly  supplied  with  individual  muscles,  in  which 
there  is  a  remarkable  correspondence  between  hand  and  foot, 
and  that  too  in  spite  of  the  loss  of  independent  action  in  the 
case  of  the  little  toe.  This  fact  of  the  rich  supply  of  muscles 
to  the  marginal  digits,  I  and  V,  is  made  much  of  by  supporters 
of  the  theory  of  the  pra-pollex  and  post-minimus,  theoretical 
digits  that  may  have  once  existed  at  either  end  of  the  present 
series  of  five.  The  muscles  in  question  are  interpreted  as  the 
musculature  of  these  extra  digits,  remaining  after  the  loss  of 
the  skeletal  parts  to  which  they  were  originally  attached.  It  is 
noteworthy  also  that  the  opponens  hallucis  is  absent  in  man 
and  has  to  be  supplied  in  the  diagram  from  the  orang  and  other 
apes  in  which  it  is  present,  and  that  a  similar  loss  or,  at  least, 
lack  of  individuality,  is  observable  in  the  appearance  of  the 
little  toe,  two  further  regressive  characters  suggestive  of  a 
slight  simplification  through  reduction. 

The  subject  of  the  homology  of  the  limbs  cannot  be  complete 
without  reference  to  the  various  methods  of  comparison  which 
have  been  proposed  by  numerous  investigators,  and  which 
depart  more  or  less  radically  from  the  one  given  here.  Thus 
the  torsion  to  which  the  limbs  have  been  plainly  subjected  ap- 
pears to  many  a  hindrance  to  a  direct  comparison  of  similar 


THE    MUSCULAR    SYSTEM 


243 


parts  as  given  above  and  leads  them  to  make  the  comparison 
in  other  ways ;  thus,  in  the  earliest  of  these  theories,  more  than 
a  century  ago,  the  right  arm  was  compared,  not  with  the  right- 
leg,  but  with  the  left;  the  thumb  became  thus  the  homologue 


D 


FIG.  65.     Diagrams  explanatory  of  various  theories  of  limb  homology. 

(A)  Syntropist  theory, — members  of  the  same  side  homologized.  (B)  Antitropist 
theory, — members  of  opposite  sides  homologized.  (C)  Homology  between  the  spinal 
nerves  involved  in  the  antitropist  theory.  The  cervical  and  dorsal  nerves  going 
posteriorly,  are  compared  with  the  lumbo-sacral  nerves  going  anteriorly.  Thus,  the 
fourth  cervical  nerve  (C4)  is  the  homolog  of  the  second  sacral  (S2),  and  so  on.  (D) 
Theory  of  FOLTZ,  1863.  The  first  digit  of  each  limb  is  bivalent,  and  the  equivalent 
of  digits  4+5  of  the  other.  (E)  Theory  of  EISLER,  1895.  Here  the  relation  is  a.n- 
titropic,  but  the  homology  applies  to  the  three  inner  digits  only  in  each  member, 
leaving  no  homologue  for  digits  4  and  5  in  each  case. 


of  the  little  toe,  radius  was  compared  with  fibula,  and  ulna 
with  tibia  (Fig.  65,  B).  That  this  theory,  fantastic  as  it  may 
seem,  is  not  merely  a  vague  speculation,  but  one  to  the  aid 


244  HISTORY   OF   THE    HUMAN    BODY 

of  which  many  facts  may  be  invoked,  is  shown  by  its  per- 
sistence in  one  form  or  another  even  to  the  present  day. 

In  fact,  the  theorists  on  the  subject  of  limb  homology  have 
been  well  divided  into  two  schools,  syntropists  and  antitropists, 
the  former  making  a  direct  comparison  of  the  limbs  of  the  same 
side,  with  the  digits  in  their  usual  order,  the  latter  changing 
the  order  either  by  reversal,  by  comparing  the  limbs  upon 
opposite  sides  of  the  body,  or  by  some  other  unusual  means. 
Of  this  there  is  every  possible  variation ;  one  theory  considers 
in  the  first  place  the  limbs  of  the  same  side  to  be  the  symmetri- 
cal equivalent  of  each  other,  and  that  thus  the  ulna  is  the 
homologue  of  the  tibia  and  the  radius  that  of  the  fibula,  and 
considers  also  the  three  radial  fingers  to  be  the  equivalent  of 
the  three  tibial  toes,  but  in  the  reverse  direction,  as  indicated 
in  the  diagram  at  E.  This  leaves  the  two  outer  digits  of  each 
member  without  correspondence  in  the  other.  Another  theory 
compares  the  digits,  also  in  the  reverse  order,  but  considers  both 
the  thumb  and  the  great  toe  bivalent,  that  is,  equal  to  two 
digits,  and  thus  compares  each  with  two  other  digits  of  the 
other  member  D.  This  comparison  of  the  digits  in  the  reversed 
direction,  however,  when  carried  to  its  conclusion,  leads  also 
to  the  homologizing  also  in  the  reversed  direction,  of  the  spinal 
nerves  that  supply  the  limbs.  Thus  the  nerves  of  the  brachial 
plexus  proceeding  posteriorly,  must  be  the  homologues  of  the 
nerves  of  the  lumbo-sacral  plexus,  proceeding  anteriorly,  as  in 
C.  Perhaps  the  most  recent  of  the  theories  in  which  there 
is  a  reversal  of  any  part  is  one  in  which  the  limbs  of  the  same 
side  are  taken  for  the  comparison,  and  in  the  normal  position 
as  far  as  the  knees,  but  which  assumes  that  in  the  distal  portion 
there  has  been  a  torsion  of  both  arm  and  leg,  thus  causing  the 
original  extensor  muscles  to  become  flexors  and  vice  versa. 
This  homologizes  the  flexors  of  the  upper  arm  with  the  exten- 
sors of  the  thigh,  but  allows  in  the  distal  portion  a  direct  com- 
parison of  the  flexors  with  flexors  and  extensors  with  ex- 
tensors. 

As  already  suggested,  the  theories  just  enumerated  are  not 
mere  vague  surmises,  but  rest  in  most  cases  upon  careful  study 


THE    MUSCULAR   SYSTEM  245 

of  the  anatomical  details;  as  they  are  not  in  accord  with  one 
another,  however,  they  cannot  all  be  right,  and  the  remarkable 
degree  of  correspondence  in  bones  and  muscles,  not  merely  in 
the  salamanders,  but  also  in  many  mammals,  including  man,  a 
correspondence  that  is  obvious  and  easily  apparent,  is  a  strong 
argument  in  favor  of  a  natural  syntropic  comparison,  as  given 
here.  The  embryological  history,  moreover,  so  far  as  it  is 
given,  shows  no  sign  of  such  a  torsion  or  reversal  as  is  de- 
manded by  the  antitropists,  but  presents  as  the  first  stage  of  the 
fore  and  hind  limb,  two  pairs  of  lateral  flaps,  each  with  a 
cranial  and  a  caudal  border  and  a  dorsal  (outer)  and  a  ventral 
(inner)  surface.  Of  these  the  cranial  border  becomes  respec- 
tively the  radial  and  tibial  side  of  the  future  limb,  the  caudal 
border  the  ulnar  and  fibular.  The  muscles  of  the  original 
dorsal  and  ventral  surfaces  remain  in  their  primary  position 
and  may  be  compared  in  the  two  limbs;  in  the  distal  portion 
the  dorsal  muscles  become  extensors,  the  ventral,  flexors,  in 
each  limb.  The  embryological  history  thus  furnishes  a  definite 
proof  in  favor  of  the  hypothesis  of  syntropism,  or  that  of  direct 
comparison,  limb  with  limb,  in  the  normal  position,  and  this 
theory  is  espoused  at  the  present  time  by  the  majority  of  in- 
vestigators. 

The  visceral  musculature  differs  from  the  axial-appendicular, 
thus  far  considered,  in  its  derivation  from  the  ventral  portions 
of  the  mesoderm,  that  is,  from  the  hypomeres  instead  of  from 
the  epimeres.  The  skeletal  parts  writh  which  it  is  associated 
are  those  of  the  visceral  arches  and  their  derivatives,  including 
the  jaw  and  the  hyoid,  and,  in  the  higher  forms  the  numerous 
cartilages  of  the  larynx  and  the  auditory  ossicles.  This  system 
has  thus  its  most  extensive  though  perhaps  not  its  most  special- 
ized development  among  the  fishes,  for  here  the  gill-arches  are 
functional  and  need  to  be  regulated  by  systems  of  levators, 
depressors,  constrictors,  dilatators  and  so  on,  which  often  attain 
a  high  degree  of  complexity.  In  the  amphibians,  where,  in 
spite  of  the  existence  of  gill-slits,  at  least  in  the  larva,  there  is 
tut  little  need  of  controlling  the  movements  of  the  separate 
arches  so  precisely,  the  visceral  musculature  appears  in  a 


246 


HISTORY    OF    THE    HUMAN    BODY 


greatly  simplified  form,  and  the  few  muscles  that  persist  enter 
into  the  service  of  aerial  respiration  and  regulate  the  opening 
and  closing  of  the  pharyngeal  cavity  and  the  larynx.  Among 
them  appear  two  well-defined  series  of  muscles,  the  one  dorsal 
and  the  other  ventral  to  the  visceral  arches,  that  act  respectively 
as  levators  and  depressors  of  those  parts.  Their  condition  in 
urodeles,  together  with  a  diagram  representing  an  hypothetical 


FIG.  66.    Diagrams  of  primitive  visceral  muscles. 

(A)  Typical  form,  hypothetical.  (B)  Condition  based  upon  that  of  the  urodele 
Siren,  with  a  few  details  supplied  from  Necturus. 

I-VII&,  levators  of  the  arches;  I-VIIv,  depressors  of  the  arches;  m,  mandibular 
arch;  h,  hyoid  arch;  b\  to  b7,  branchial  arches;  t,  trigeminus;  fa,  facialis;  gl,  glosso- 
pharyngeus;  v,  to  v4  vagus  (pneumogastric)  elements;  x,  temporalis;  y,  masseter;  z, 
digastricus;  la  —  1-4,  levatores  arcuum;  dl,  dorso-laryngis  and  dorso-trachealis;  a,  inter- 
mandibularis  anterior;  c,  intermandibularis  posterior;  d,  hyo-pharyngeus,  anterior  por- 
tion; e,  hyo-pharyngeus,  posterior  portion;  /,  laryngei. 


ancestor    from    which    may   have    been    derived,    are    given 
in  Fig.  66. 

In  the  diagram  A  the  seven  visceral  arches,  including  the 
mandible,  are  given  in  order,  representing  as  many  somites, 
with  their  motor  nerve  supply.  For  the  first  or  mandibular 
somite  this  latter  is  the  mandibular  branch  of  the  trigeminus; 
for  the  second  or  hyoid,  the  facialis;  for  the  third,  the  glosso- 


THE    MUSCULAR   SYSTEM  247 

pharyngeus;  and  for  the  remaining  four,  a  like  number  of 
branches  from  the  vagus,  which  is  a  complex  of  several  original 
elements.  The  dorsal  muscles  attached  to  the  arches  are 
Icvators,  the  ventral  depressors.  In  the  second  figure,  B,  is 
shown  the  actual  condition  in  urodeles,  the  derivation  of  which 
from  the  first  is  obvious. 

Beginning  with  the  levator  series,  the  first  becomes  the  equiv- 
alent of  the  adductor  mandibulce  of  fishes,  here  differentiated 
into  temporalis  and  masseter,  the  muscles  of  mastication.  The 
second,  having  its  primary  connection  with  the  hyoid  arch,  be- 
comes also  attached  to  the  mandible,  but  in  such  a  way  that  it 
opens  it,  thus  acting  as  the  antagonist  to  the  first.  This  mus- 
cle is  usually  referred  to  as  the  "  digastric,"  a  name  taken  from 
human  anatomy,  but  it  is  probably  homologous  with  the  pos- 
terior belly  alone  of  the  mammalian  muscle  of  the  same  name. 

The  next  four  muscles  are  those  associated  with  the  first  four 
gill-arches,  and  function  in  the  lower  urodeles  and  in  the  larvae 
of  the  more  specialized  ones  as  the  levatores  arcuum;  the  next 
and  last  belongs  plainly  in  the  same  series,  but  as  its  arch  has 
become  specialized  as  the  primary  laryngeal  cartilage  [Chapter 
V],  it  extends  ventrally  to  meet  it.  On  account  of  this  rela- 
tionship it  has  received  the  name  of  dorso-laryngeus. 

The  ventral  series  consists  of  flat  sheets,  arising  from  the 
mid-ventral  line,  where  they  meet  in  pairs.  Of  these,  the  first 
two,  the  intcrmandibulares,  anterior  and  posterior,  form  the 
muscular  floor  of  the  mouth  and  are  attached  respectively  to 
the  mandible  and  the  hyoid.  Of  the  next  two,  those  associated 
with  the  two  first  gill-arches,  there  is  no  trace,  and  the  next, 
the  fifth,  is  present  only  in  Necturus  and  its  ally,  Proteus,  the 
lowest  of  the  urodeles.  The  sixth,  under  the  often  inappro- 
priate name  of  hyo-laryngeus,  is  generally  present,  and  the 
seventh,  stretching  between  the  two  lateral  laryngeal  cartilages, 
becomes  a  set  of  true  laryngeal  muscles,  the  dorsal  and  ventral 
laryngei  (laryngcus  dorsalis  and  laryngeus  ventralis). 

Above  this  stage  the  further  phylogenetic  history  of  the 
visceral  muscles  has  been  followed  only  in  part,  and  the  con- 
clusions drawn  are  those  which  are  the  most  obvious. 


248  HISTORY   OF   THE    HUMAN    BODY 

The  two  masticatory  muscles,  temporalis  and  masseter,  occur 
in  all  higher  forms  and  are  homologous  throughout,  save  that 
two  farther  slips,  the  pterygoideus  externus  and  interims,  be- 
come differentiated  from  them,  probably  from  the  original 
masseter.  In  mammals  a  small  slip  from  the  pterygoideus 
internus,  becomes  the  tensor  tympani  of  the  middle  ear.  The 
second  levator  becomes  associated  with  a  muscular  slip  from 
the  first  depressor,  intermandibularis  anterior,  and  forms  the 
digastricus  of  mammals,  the  two  elements  being  united  by  a 
tendon.  The  diploneuric  character  of  this  muscle,  that  is,  the 
innervation  of  the  anterior  belly  from  the  trigeminus  and  that 
of  the  posterior  from  the  faciaKs,  receives  thus  an  explanation. 
A  portion  of  the  posterior  belly,  that  is,  of  the  second  levator, 
becomes  separated  from  it  in  reptiles,  and  follows  the  stapes 
into  the  middle  ear,  whence  it  becomes  the  stapedius  muscle, 
innerved  by  a  special  branch  of  the  facialis.  The  ventral  mus- 
cles of  these  same  first  two  segments  are  perpetuated,  the  first 
in  part  as  the  anterior  belly  of  the  digastricus  just  mentioned 
and  in  part  as  the  mylo-hyoideus;  the  second  as  the  stylo- 
hyoideus.  Beyond  this,  however,  the  history  is  not  clear.  In 
the  previous  chapter  it  was  shown  that  the  various  gill-arches, 
beginning  posteriorily,  become  associated  with  the  original 
pair  of  laryngeal  cartilages  to  form  the  complicated  larynx  of 
higher  forms,  but  whether  the  muscular  elements  primarily 
\  associated  with  them  assist  in  the  formation  of  the  musculature 
^  of  the  final  organ,  or  whether  this  musculature  is  derived 
entirely  from  the  muscles  primarily  belonging  to  the  seventh 
arch,  that  is,  dorso-laryngeus  and  the  laryngei,  cannot  yet  be 
definitely  stated. 

The  musculature  of  the  tongue,  especially  its  extrinsic 
muscles,  such  as  hyo-glossus,  genio-glossus,  stylo- glossus,  etc., 
is  probably  derived  from  the  visceral  muscles,  but  here  another 
element  is  introduced,  and  that  is  the  muscular  layer  of  the 
anterior  end  of  the  alimentary  canal,  which,  although  of 
mesenchymatous  origin,  and  primarily  composed  of  unstriated 
cells,  involuntary  in  their  action,  are  yet  capable  of  acquiring 
striae  and  of  becoming  at  least  semi-voluntary.  From  this 


THE    MUSCULAR   SYSTEM  249 

layer  are  derived  the  pharyngeal  constrictors,  and  it  is  probable 
that  the  intrinsic  muscular  fibers  which  make  up  the  mass  of 
the  tongue  and  known  as  the  lingualis,  come  from  the  same 
source. 

Superficial  to  the  muscular  systems  already  described,  and 
lying  directly  beneath  the  integument  there  are  found  in  many 
vertebrates  muscular  elements,  usually  in  the  form  of  sheets 
or  layers,  and  connected  with  the  integument,  which  thus  ac- 
quires locally  some  power  of  movement.  These  muscles  form 
what  may  be  conveniently  termed  the  integumental  system, 
although  there  are  included  here  contributions  from  several 
wholly  unrelated  systems,  independently  developed  in  the  dif- 
ferent groups  of  animals  to  subserve  special  functions  and 
therefore  restricted  in  their  occurrence.  These  integumental 
muscular  elements  have  arisen  from  whatever  preexisting 
muscles  happen  to  be  adjacent  to  the  location  where  such  a  part 
is  needed,  and  thus  they  may  be  in  their  origin  either  axial,, 
visceral  or  appendicular,  or  may  represent  a  combination  of 
these  systems.  They  usually  possess  at  one  end  a  firm  attach- 
ment to  some  skeletal  part  or  at  least  to  skeletal  muscles,  while 
at  the  other  end,  or  perhaps  along  an  extended  surface,  they 
adhere  to  the  inner  side  of  the  integument,  thus  furnishing 
the  skin  area  involved  with  the  degree  of  motion  required. 

In  both  birds  and  mammals  certain  shoulder  muscles  furnish 
an  important  contribution  to  the  integumental  system,  but, 
as  would  be  expected,  the  two  cases  are  totally  independent 
of  one  another.  In  birds  the  integumental  area  involved  is 
the  patagium  or  web,  extending  across  the  angles  of  the 
axilla  and  elbow  and  increasing  the  resisting  surface  of  the 
wing.  This  is  regulated  by  a  series  of  patagial  muscles,  strictly 
integumental  in  their  relations,  but  derived  from  the  various 
muscles  of  shoulder  and  arm ;  of  these  the  most  important 
are  M.  propatagialis,  derived  from  the  anterior  portion  of  the 
pectoralis,  and  an  associated  slip  from  the  biceps. 

In  mammals  an  extensive  layer,  derived  from  latissimus  and 
pectoralis,  spreads  over  the  side  of  the  body,  and  in  some 
cases  the  two  extend  to  the  mid-dorsal  and  mid-ventral  lines,. 


250 


HISTORY   OF   THE    HUMAN    BODY 


encasing-  the  trunk  in  a  sub-cutaneous  muscular  sheet.  This 
is  the  panniculus  carnosus,  and  is  primarily  employed  in  mov- 
ing and  wrinkling  the  skin  as  a  defense  against  insects.  In 
the  monotremes  the  portion  derived  from  the  pectoralis  ex- 
tends over  the  entire  ventral  aspect  of  the  body,  and  where 
it  meets  the  marsupial  pouch  and  the  cloacal  orifice  forms 


D 


FIG.  67.     Phylogenesis  of  the  panniculus  carnosus.     [After  TOBLER.] 

(A)  Macropus  bennett  (kangaroo).  (B)  Cynocephalus  hacmadryas.  (C)  Cerco- 
pithecus  sabaeus.  (D)  Cercopithecus  cephus. 

from   its   fibers   certain   more   specialized   slips   to    serve   as 
sphincters  (sphincter  marsupii  and  sphincter  cloacce). 

A  panniculus  carnosus,  perhaps  here  mainly  a  contribution 
from  the  latissimus,  is  also  present  in  marsupials  (Fig.  67,  A) 
and  covers  the  flanks  with  fibers  that  converge  to  an  insertion 
into  the  humerus.  From  these  it  is  directly  continued  to  the 
Insectivora  and  Carnivora,  and  to  other  Orders  of  mammals. 
Its  action  is  seen  in  the  shaking  of  the  skin  of  a  wet  dog  or 
the  twitching  along  the  outer  portion  of  the  legs  of  horses 
and  cattle  when  these  surfaces  are  stimulated  by  the  bite  of 
an  insect.  In  the  lower  primates,  the  panniculus  appears  as 


THE    MUSCULAR    SYSTEM  251 

a  broad  sheet  upon  each  side,  much  as  in  marsupials  (Fig. 
67,  B),  but  within  this  Order  it  is  seen  to  separate  into  axil- 
lary and  inguinal  portions  (Fig.  67,  C  and  D)  and  in  the  an- 
thropoids, the  former  alone  remains,  much  reduced  in  size 
(Fig.  68).  In  man  there  appear  to  be  normally  no  traces  of 


FIG.  68.  Anterior  remnant  of  the  panniculus  carnosus,  "  achselbogen," 
in  the  gorilla.  [After  TOBLER.] 

pmj,  pectoralis  major;  fob,  coraco-brachialis  fascia;  Id,  latissimus  dorsi;  pa, 
"  pectoralis  quartus,"  a  part  of  the  panniculus;  x,  tendinuous  fibers  from  the 
latter. 

this  muscle,  but  there  occurs  occasionally  a  system  of  slips  in 
the  axillary  region,  the  axillary  arch  ("Achselbogen")  asso- 
ciated with  both  latissimus  and  pectoralis  and  very  variable  in 
appearance,  a  typical  characteristic  of  a  rudiment.  In  asso- 
ciation with  this,  there  occasionally  develops  a  more  posterior 
pectoralis  slip,  the  pectoralis  abdominis,  a  relic  from  a  remote 
part.  Still  another  rudiment  of  the  panniculus  system  is  seen 


252  HISTORY   OF   THE   HUMAN   BODY 


B 


FIG.  69.     Two  cases  of  axillary  panniculis  rudiments  in  man. 


;,  GEHRY]-    pmj'    Pectoralis    major;    pmn,    pec- 

minor;  d  deltoid;  Id,  latissimus;  pa,  •«  pectoralis  quartus,"  a  part  of  the 
panmculus;  x,  the  definite  ««  achselbogen  »;  st,  the  sternalis,  a  rare  muscular 
anomaly,  also  a  part  of  the  panniculus. 


THE    MUSCULAR    SYSTEM  253 

in  the  stcrnalis  muscle,  an  element  of  very  rare  occurrence  and 
proven  to  belong  here  by  its  occasional  relationship  with  both 
pectoralis  abdominis  and  the  elements  of  the  axillary  arch, 
as  well  as  by  its  innervation  from  the  anterior  thoracic  nerve, 
in  common  with  the  foregoing  (Fig.  69).  The  st emails 
muscle  has  been  recently  shown  to  occur  much  more  frequently 
in  the  Japanese  than  in  Europeans  (13  per  cent,  against  4 
per  cent. ) .  It  lies  superficial  to  the  pectoralis  major,  and  when 
well  developed  may  be  so  contracted  as  to  be  plainly  visible 
from  the  exterior. 

Another  system  of  integumental  muscles  is  derived  from  the 
visceral  musculature  and  appears  in  its  simplest  form  in  the 
sphincter  colll  of  amphibians,  reptiles  and  birds,  and  is  itself 
a  direct  descendant  of  a  selachian  muscle,  the  superficial  con- 
strictor. The  fibers  of  this  sheet  enwrap  the  neck  region  and 
in  turtles  and  birds  the  muscle  is  well  developed  and  covers  the 
entire  neck.  In  mammals  this  sheet  differentiates  into  two 
layers,  a  more  extensive  superficial  layer,  the  platysma, 
and  a  smaller  and  deeper  layer,  which  retains  the  original 
name  of  sphincter  colll.  The  fibers  of  these  two  sheets  run 
primarily  at  right  angles  to  one  another,  those  of  the  platysma 
being  directed  upwards  and  towards  both  snout  and  ear,  those 
of  the  sphincter  in  more  nearly  the  original  direction  across 
and  around  the  neck. 

In  following  the  phylogenetic  series  through  marsupials 
and  lemurs  to  primates,  a  considerable  extension  of  both  of 
these  layers  over  the  face  and  head  is  noticed,  and  as  they 
meet  the  eyes,  nose,  ears,  and  lips  there  is  seen  a  pronounced 
'tendency  to  form  special  slips  for  the  regulation  of  these  parts t 
a  tendency  precisely  similar  to  that  of  the  ventral  panniculus 
in  the  case  of  the  marsupial  pouch  and  the  cloaca  of  the  mono- 
tremes.  There  is  thus  formed  the  extensive  system  of  facial 
muscles,  often  termed  the  "mimetic"  muscles,  which  become 
so  highly  differentiated  in  the  apes  and  in  man,  and  this  grad- 
ual differentiation  can  be  clearly  followed  in  the  phylogenetic 
series  (Fig.  70). 

The  superficial  sheet  or  platysma    extends  upwards  across 


254 


HISTORY    OF    THE    HUMAN    BODY 


Occ. 


Tri. 


AlLttt 


Hat 


Front 


Plat. 


FIG.  70.  Mimetic  muscles  in  Ateles  (a  South  American  monkey). 
[After  RUGE.] 

(A)    Superficial   layer.      (B)    Deep   layer. 

Front,  frontalis;  Au.  sup,  auricularis  superior;  O.  p,  orbicularis  palpebrarum;  Zyg, 
zygomaticus;  Nas,  nasalis;  A,  I,  s,  auriculo-labialis  superior;  A.  I.  i.,  auriculo-labialis 
inferior;  Au.  ant,  auricularis  anterior;  Au.  post,  auricularis  posterior;  Antitr,  anti- 
tragicus;  Plat,  platysma  myoides;  Occ,  occipitalis;  Tri,  triangularis;  MX.  lab,  maxillo- 
labialis;  O.  or,  orbicularis  oris;  Bucc,  buccinator. 


THE    MUSCULAR    SYSTEM  255 

the  side  of  the  neck,  and,  reaching  the  ear,  divides  into  two 
sheets,  dorsal  and  ventral.  The  dorsal  sheet,  auriculo-occipit- 
alls,  subdivides  once  more  and  furnishes  the  auricularis  pos- 
terior [retrahens  auris~]  and  the  occipitalis  (i.e.,  the  occipital 
portion  of  the  "  occipito-frontalis  "  of  human  anatomy),  and 
from  the  latter  are  derived  the  intrinsic  muscles  of  the  dorsal 
surface  of  the  ear-flap,  rudimentary  in  man.  The  ventral  or 
facial  portion  gives  off  along  the  sides  of  the  mandible  the  two 
slips,  levator  menti  and  quadratus  labii  inferioris  [depressor 
labii  inferioris'] ,  which  latter  becomes  attached  to  the  bone,  and 
is  continued  over  the  face  as  M.  sub-cutaneus  faciei.  The  ulti- 
mate differentiations  of  this  latter  portion  are  quite  complex 
and  concern  three  portions  into  which  the  sheet  divides  itself. 
Of  these  an  auriculo-labialis  inferior  furnishes  the  intrinsic 
muscles  upon  the  ventral  or  forward  surface  of  the  ear-flap 
and  an  auriculo-labialis  superior  differentiates  into  the  zygo- 
maticus  [major],  the  orbicularis  oculi  [palpebrarum]  and  the 
levators  of  the  lip  and  side  of  the  nose.  Finally  a  third  element, 
the  or  bit  o -auricularis,  furnishes  two  of  the  extrinsic  ear  mus- 
cles, auricularis  anterior  and  superior  [attrahens  and  attol- 
leus  auris~\,  and  the  frontalis,  which  in  the  apes  comes  nearly 
in  contact  with  the  occipitalis  previously  mentioned,  the  two 
becoming  connected  by  a  fascia.  The  gradual  lifting  of  the 
cranial  dome  and  the  formation  of  a  forehead,  culminating 
in  man,  spreads  apart  the  two  muscular  elements  of  this 
occipitalis-frontalis  sheet  and  extends  the  intervening  fascia 
to  become  the  gale  a  aponeurotica,  so  extensive  in  Man.  That 
portion  of  the  platysma  which  covers  the  sides  of  the  neck 
in  Man  remains  in  its  original  undifferentiated  condition,  and, 
although  quite  variable  in  its  occurrence  and  in  the  control 
over  it,  is  yet  often  capable  of  throwing  the  skin  into  longi- 
tudinal folds,  its  original  function. 

The  deeper  layer,  the  sphincter  colli  proper,  extends  also 
to  the  face,  but  is  mainly  confined  to  the  region  about  the 
mouth,  where  it  gives  rise  to  orbicularis  oris,  caninus  [levator 
anguli  oris],  buccinator  and  the  intrinsic  muscles  of  the  nose. 

The  original  sphincter  colli,  as  found  in  reptiles  and  mono- 
tremes,  lies  within  the  province  of  the  seventh  cranial  nerve 


256  HISTORY    OF    THE    HUMAN    BODY 

and  is  wholly  supplied  from  this  source.  Exhibiting  a  superb 
example  of  the  constancy  of  a  muscular  innervation,  the 
branches  of  this  nerve  expand  and  differentiate  into  the  mus- 
cle which  it  supplies,  and  with  the  migration  of  the  latter 
to  the  face  there  comes  also  the  nerve;  and  it  thus  happens 
that  this  element,  originally  the  motor  nerve  of  the  hyoid 
region,  comes  to  be  called  the  "  facialis,"  corresponding  to  the 
region  in  which  it  is  met  with  in  Man,  whose  anatomy  first  at- 
tracted especial  attention. 

That  this  system  of  facial  muscles  was  primarily  developed 
for  the  purpose  of  regulating  the  orifices  of  the  mouth  and 
the  organs  of  special  sense  there  can  be  no  question,  but 
the  high  degree  of  specialization  attained  in  the  higher  pri- 
mates suggests  a  totally  distinct  function,  that  of  communi- 
cation of  the  moods  of  the  animal  to  its  associates,  that  is,  a 
language.  That  some  outward  expression  for  the  developing 
power  of  thought  should  show  itself  pan  passu  with  the  de- 
velopment of  brain  was  to  be  expected,  and  it  appears  that 
at  about  the  point  at  which  this  muscular  differentiation  became 
apparent,  that  is,  among  the  lemurs,  the  various  cries 
produced  by  the  larynx  became  insufficient  and  were  supple- 
mented by  the  development  of  mimetic  muscles,  through  the 
medium  of  which  far  more  subtle  shades  of  meaning  could  be 
expressed.  For  a  time,  therefore,  in  the  anthropoid  precur- 
sors of  man,  both  forms  of  intercommunication  must  have  ex- 
isted side  by  side,  and  have  been  of  about  equal  value  or  with 
some  advantage  in  favor  of  the  mimetic  muscles,  as  in  the 
apes  of  the  present  day;  but  when,  by  the  shortening  of  the 
snout  and  the  consequent  flattening  of  the  dental  arcade  a 
greater  differentiation  of  articulate  sounds  became  possible, 
these  latter  became  more  and  more  employed  as  the  better 
medium  of  intercommunication,  and  the  help  of  the  facial 
muscles  became  less  and  less  necessary.  Corresponding  to 
this  change,  many  of  the  mimetic  muscles,  such  as  those  of  the 
ears,  the  nose,  and  the  scalp,  show  in  man  less  power  than 
in  the  apes,  while  those  of  the  cheeks  and  lips,  employed  as 
auxiliary  to  the  production  of  articulate  sounds,  have  reached 
a  still  higher  degree  of  development. 


CHAPTER   VII 
THE   DIGESTIVE   AND    RESPIRATORY    SYSTEMS 

"  Wie  in  jeder  Wissenschaft  aus  den  Thatsachen 
Schliiss  sich  ergeben,  welche  das  werthvollste 
Ergebnis  der  Forschung  darstellen,  so  sind  auch  fur 
die  vergleichende  Anatomic  die  geistige  Verwerthung 
der  Thatsachen  durch  ihre  Verkniipfung  das  wissen- 
schaftliche  Ziel.  Was  kann  es  nutzen,  unendlich  die 
Organisation  betreffende  Erfahrungen  zu  sammeln, 
wenn  daraus  nicht  eine  Einsicht  in  jene  erwachst,  ihr 
allmahliches  Werden  verstandlich  wird,  indem  es  sich 
in  mannigfachen,  aber  auseinander  hervorgegangenen 
Zustanden  darstellt,  die  ihre  Verwandtschaft  unter 
einander  in  der  Organisation  zum  Ausdruck  kom~ 
men  lassen." 

CARL   GEGENBAUR,  Lehrbuch  d.   vergl.  Anat. 

1898  ed.,  p.  27. 

THE  first  step  in  the  evolution  of  the  Metazoa  from  pro- 
tozoan cell  colonies,  that  is,  the  procedure  which  initiated  the 
transformation  of  a  colony  of  similar,  one-celled  organisms 
into  a  single  organism  of  many  cells,  was  the  inpushing  of  its 
walls  at  a  given  point,  resulting  in  the  formation  of  a  two- 
layered  cup,  the  gastrula.  From  that  moment  on,  the  inner 
and  outer  cells  become  placed  in  a  .different  position  relative 
to  the  entire  organism,  and  were  thus  subjected  to  different 
experiences.  The  outer  layer  was  interposed  between  the 
organism  and  the  external  world;  the  inner  dealt  entirely 
with  the  material  received  into  the  cavity  formed  by  it  and 
used  for  food.  It  is  obvious  that  this  difference  of  experience 
would  result  in  a  physical  differentiation  of  the  cells,  and  such 
was,  indeed,  the  case.  The  cells  of  the  outer  layer,  the  ecto- 
derm, became  in  part  protective  and  in  part  receptive  of  ex- 
ternal stimuli,  differentiations  later  to  result  in  the  formation 
of  an  exo-skeleton  and  a  nervous  system ;  those  of  the  inner 
layer,  the  endoderm,  developed  the  power  of  obtaining,  ab- 


258  HISTORY   OF   THE    HUMAN    BODY 

sorbing,  and  assimilating  the  nutritive  qualities  of  the  food, 
and  thus  formed  the  digestive  cavity,  the  first  portion  of  the 
organism  to  differentiate  as  a  distinct  system.  This  digestive 
/cavity,  or  gastroccele,  remains  in  the  lower  invertebrates  as  a 
blind  cavity  with  but  a  single  opening,  and  first  among  the 
worms  (Vermes),  it  becomes  converted  into  a  complete  canal 
by  the  formation  of  an  anal  orifice,  thus  obviating  the  necessity 
of  employing  the  same  orifice  for  both  the  intaking  and  the 
expulsion  of  the  contents  of  the  cavity. 

A  further  advance  in  the  development  of  the  endodermic 
portion  of  the  organism  is  seen  in  the  higher  invertebrates 
(articulates,  echinoderms,  etc.),  and  in  the  vertebrates,  where 
certain  lateral  diverticula  become  divided  off  from  the  primary 
alimentary  canal,  and  form  a  definite  body  cavity,  the  cceloin, 
so  that  the  ultimate  alimentary  canal  of  these  animals  is  but 
a  part  of  the  canal  of  the  lower  organisms.  In  vertebrates 
the  canal  suffers  a  still  farther  loss  by  the  formation  and  later 
separation  of  the  notochord.  Another  departure  from  the 
primary  condition  is  seen  in  the  mouth  and  anus  of  vertebrates, 
which  are  shown  by  their  development  not  to  be  homologous 
with  the  similarly  named  cavities  of  lower  forms  but  new 
formations,  involving  other  morphological  relations,  and 

,  formed  by  contributions  from  the  ectoderm.  In  the  develop- 
ment of  many  invertebrates  the  primary  mouth  of  the  gastrula 
becomes  the  permanent  one  of  the  adult  organism  and  an 
anus  is  formed  by  continuing  the  blind  end  of  the  gastrular 
invagination  until  it  meets  the  surface  ectoderm  at  a  point 
opposite  that  of  the  mouth ;  in  the  vertebrate  embryo,  how- 
ever, the  gastrular  mouth  lies  postero-dorsally  with  reference 
to  the  future  animal  and  thus  bears  no  relation  to  either 

,  mouth  or  anus  of  the  perfected  form  (Fig.  71,  A).  During 
the  development  of  the  nervous  system,  however,  there  comes 
a  curious  and  inexplicable  connection  between  the  lumen 
of  the  neural  tube  and  the  gastrular  mouth,  which  effects  a 
temporary  connection  between  this  cavity  and  that  of  the 
gastrocoele  through  the  so-called  neur enteric  canal  (Fig.  71, 
B),  but  this  connection  is  only  transitory  and  the  entire  struc- 


THE    DIGESTIVE   AND    RESPIRATORY   SYSTEM     259 

ture  soon  disappears,  leaving  the  gastroccele  as  a  closed  sac,' 
with  neither  oral  nor  anal  orifices.  These  are  formed  secon- 
darily through  inpushings  of  the  ectoderm,  the  blind  ends  of 
which  come  in  contact  with  the  endoderm  and  later  break 
through  at  the  point  of  contact,  thus  completing  the  canal 
(Fig.  71,  B,  in  and  an).  The  functional  alimentary  canal 


B 


FIG.  71.  Diagrams  showing  the  formation  of  the  vertebrate  alimentary 
canal  and  nerve  cord,  and  the  early  relation  between  them. 

(A)  Early  embryo,  immediately  after  the  gastrular  stage,  based  on  Amphioxus. 
Compare  this  with  Fig.  13  (c).  (B)  later  stage,  based  on  the  embryo  of  the  frog. 

g,  gastroccele  (=cavity  of  alimentary  canal);  n,  neurocosle  (=<cavity  of  the 
neural  tube);  ne,  neurenteric  canal;  np,  neuropore;  b,  blastopore;  an,  developing 
proctodseum;  m,  point  where  the  stomatodaeal  invagination  will  take  place;  d,  liver 
invagination;  h,  heart;  nc,  notochord. 

thus  comes  to  be  formed  of  three  elements,  an  anterior  ecto- 
dermic  one,  the  stomatodccum,  a  middle  endodermic  one,  the 
mesodcntin,  and  a  posterior  portion,  also  ectodermic,  the 
proctod&um.  In  the  articulates  (crustaceans,  insects,  and 
spiders),  in  \vhich  the  alimentary  canal  is  formed  in  much 
the  same  way,  the  proportion  of  the  functional  digestive  tract 


260  HISTORY    OF    THE    HUMAN    BODY 

formed  by  stomato-  and  proctodseum,  and  therefore  ectoder- 
,  mic,  is  extremely  large,  and  the  mesodaeum,  though  volumi- 
nous, is  employed  mainly  in  the  formation  of  laterally  placed 
digestive  glands;  but  in  the  vertebrate  the  canal  is  mainly 
mesodaeal,  and  therefore  endodermic,  the  ectodermic  oral  and 
anal  contributions  being  much  restricted. 

With  the  exception  of  a  small  number  of  auxiliary  organs 
like  the  jaws,  teeth,  and  tongue,  the  entire  digestive  system  is 
derived  from  this  simple  tube,  and  all  the  parts  which  appear 
in  even  the  most  complicated  cases  develop  from  this  by 
.  means  of  such  mechanical  principles  as  increase  in  length,  local 
enlargement,  foldings,  outpushings,  and  inpushings,  in  short, 
such  principles  as  are  employed  in  developmental  history  every- 
where. More  than  this,  from  the  anterior  portion  of  this  canal 
there  develop  the  two  principal  respiratory  systems  of  verte- 
brates, the  branchial  or  gill  system  for  aquatic  breathing,  and 
the  pulmonary  or  lung  system  for  air.  This  close  association 
between  digestive  and  respiratory  systems  is  essentially  a  ver- 
tebrate characteristic  and  is  hardly  known  among  other  ani- 
mals, save  in  the  cases  of  Amphioxus,  the  tunicates  and  the 
Enteropneusta,  which  in  other  respects  also  show  their  close 
affinity  to  the  Vertebrata.  [See  Chap.  XII. ]  Although  so 
closely  related  anatomically,  the  digestive  and  respiratory  sys- 
tems are  best  disassociated  in  treatment  and  will  be  considered 
separately  as  far  as  possible. 

Although  essentially  and  in  its  origin  an  endodermic  organ, 
the  alimentary  canal  always  becomes  reinforced  by  other  tis- 
sues which  form  layers  outside  of  the  primary  endodermic  one. 
«  Including  the  latter  the  layers  are  usually  considered  four  in 
number,  named  in  order,  beginning  with  the  inner  one :  mu- 
cosa,  submncosa,  musculosat  and  serosa.  The  mucosa  is  the 
primary  agent  in  digestion  and  develops  glands  for  the  pro- 
duction of  the  various  necessary  digestive  juices;  it  also 
contains  a  thin  layer  of  involuntary  muscular  fibers  and  is 
permeated  with  blood  and  lymphatic  vessels  for  absorbing  and 
carrying  away  the  nutriment  when  in  a  proper  condition  for 
assimilation.  The  submucosa  is  a  thin  layer  of  connective 


THE    DIGESTIVE   AND   RESPIRATORY   SYSTEM     261 

tissue,  needed  to  give  support  to  the  more  delicate  mucous 
layer.  The  musculosa,  or  muscular  coat,  may  vary  much  in 
the  different  regions,  but  consists  typically  of  involuntary  mus- 
cular cells  arranged  in  two  layers,  circular  and  longitudinal, 
the  former  internal.  By  the  contraction  of  the  circular  layer 
the  caliber  of  the  tube  is  lessened  and  its  length  increased, 
while  by  the  contraction  of  the  longitudinal  fibers  the  tube 
is  shortened  and  thickened.  Various  combined  actions  of  these 
fibers  produce  the  peristaltic  movements  which  occur  during 
active  digestion,  and  furnish  an  important  mechanical  aid  in 
the  process.  The  serosa,  or  serous  covering  for  the  tube,  is  in 
reality  a  reflexed  portion  of  the  peritoneum,  which  lines  the 
coelom,  and  which  is  attached  in  such  a  way  that,  besides 
covering  the  canal  itself,  its  reflexions  form  broad,  supporting 
membranes  known  as  mesenteries,  which  attach  the  tube 
loosely  to  the  body  wall  and  hold  it  in  place. 

In  no  living  vertebrate  is  the  canal,  when  fully  developed, 
in  the  form  of  a  straight,  undifferentiated  tube,  but  becomes 
modified  in  several  ways.  In  the  first  place,  through  the  nor- 
mal process  of  digestion,  it  necessarily  becomes  divided  into 
regions,  each  of  which  is  devoted  to  the  performance  of  a 
certain  physiological  function,  either  mechanical  or  chemical. 
These  portions  are  furthermore  differentiated  from  one 
another  in  shape  and  size,  and  vary  from  long,  attenuated 
tubes,  to  short  and  wide  sacs;  some  grade  into  one  another 
without  definite  boundaries ;  others  are  quite  sharply  set  apart 
by  a  sudden  change  of  external  shape,  by  a  localized  restric- 
tion in  the  caliber  of  the  tube,  or  by  the  entrance  at  a  definite 
point  of  some  new  digestive  juice. 

A  second  cause  for  modification  in  the  primary  simple 
digestive  tube  lies  in  the  mathematical  law  of  the  ratio  of 
surface  to  mass,  whereby  the  surfaces  of  two  homologous 
solids  are  as  their  squares,  the  masses  as  the  cubes,  of  their 
homologous  dimensions.  If,  for  example,  an  animal  posses- 
sing a  straight  alimentary  canal  with  a  smooth  mucous  lining 
were  to  increase  to  twice  its  original  length,  its  bulk  would 
be  increased  eight 'times,  but  the  square  surface  of  the  in- 


262  HISTORY   OF    THE    HUMAN    BODY 

terior  of  the  alimentary  canal  but  four  times ;  in  other  words, 
it  would  have  but  half  as  much  digestive  power,  and  would 
be  in  danger  of  starving  were  not  some  means  employed  to 
proportionately  increase  its  digestive  surface.  As  the  physio- 
logical digestive  membrane  is  the  mucosa,  this  layer  is  the  one 
primarily  concerned  in  these  modifications,  and  increase  in  its 
surface  is  gained,  ( i )  by  lengthening  the  entire  canal  and  al- 
lowing it  to  fold  or  coil  in  some  more  or  less  definite  fashion, 
(2)  by  the  formation  of  diverticula,  or  blind  pockets,  usually 
long  and  narrow  like  the  canal  itself,  and  (3)  by  various  meth- 
ods of  folding  or  wrinkling  the  mucosa  itself,  with  or  without 
the  other  modifications.  Independently  of  the  above  law,  vari- 
ations in  the  amount  of  digestive  surface,  and  especially  in 
the  capacity  of  those  portions  of  the  canal  used  as  temporary 
reservoirs,  are  dependent  upon  the  quality  of  the  food  habitu- 
ally taken,  an  innutritions  food  requiring  a  greater  capacity 
and  probably  a  greater  mucous  area  than  a  more  concentrated 
one. 

A  third  necessary  tendency  of  the  mucosa  is  the  formation 
of  glands  for  the  elaboration  of  the  various  digestive  juices 
needed  in  the  case  of  different  kinds  of  foods  and  in  different 
stages  of  the  process;  and  in  this  are  shown  again  the  prin- 
ciples of  gland  formation  as  treated  above  in  the  case  of  in- 
tegument. Thus  there  is  a  widespread  occurrence  of  beaker 
cells  and  of  simple  tubular  glands,  which  dip  a  short  distance 
below  the  surface,  and  as  they  are  usually  placed  close  to- 
gether they  form  a  thick  mucosa,  the  thickness  of  which  is 
that  of  the  length  of  the  glands  composing  it.  In  more  com- 
plicated cases  the  glands  may  become  too  large  to  be  included 
within  the  mucosa  and  push  their  way  outward  to  the  serosa, 
beneath  which  they  appear  as  localized  swellings,  as  in  the  case 
of  the  pancreas  of  many  fishes;  a  still  farther  extension  of 
this  principle  produces  an  accessory  organ  like  the  liver  or 
like  the  pancreas  of  higher  vertebrates,  beyond  the  bounds  of 
the  alimentary  canal,  but  connected  to  it  by  one  or  more  ducts 
and  still  invested  by  the  serosa  (peritoneum). 

The  application  of  these  principles  and 'the  gradual  attain- 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     263 

ment  of  a  complicated  alimentary  canal  is  shown  by  the  com- 
parison of  various  phylogenetic  stages  (Fig.  72).  At  first  the 
canal  does  not  much  exceed  the  body  in  length,  and  its  wind- 
ings consist  of  a  few  open  curves,  although  in  actual  cases  the 
length  may  vary  as  the  quality  of  the  food,  and  it  may  thus 
happen  that  in  two  closely  allied  forms  considerable  differ- 


FIG.  72.     Comparative  diagrams  of  the  alimentary  canal. 

(A)    Fish.      (B)    Bird.      (C)    Mammal. 

I,  pharynx;  II,  oesophagus;  III,  stomach;  IV,  duodenum;  V,  intestine;  VI,  cloaca; 
g,  salivary  glands;  r,  thyreoid  gland;  in,  thymus  gland;  I,  bronchi,  leading  to  the 
lungs;  h,  liver;  ;',  pancreas;  k,  spleen;  y,  pyloric  diverticula  (in  fishes);  z,  cloacal 
diverticuia  (in  birds);  x,  intestinal  diverticulum,  the  ccecum  (in  mammals);  a,  ap- 
pendix (the  narrowed  free  end  of  the  latter  in  man).  In  B  the  stomach,  III,  is  in 
two  parts,  glandular  and  muscular;  in  C  the  intestine  is  differentiated  into  Va, 
small  intestine;  Vb,  ascending  colon;  Vc,  transverse  colon;  Vd,  descending  colon; 
and  Ve,  the  rectum. 


ence  may  be  seen  in  this  particular.  Indeed,  a  great  contrast 
in  the  length  of  the  canal  may  occur  in  various  stages  of  the 
same  animal,  as  in  the  frog,  the  tadpole  of  which  possesses 
a  spiral,  much  coiled  intestine,  while  that  of  the  adult  shows 


264 


HISTORY   OF    THE    HUMAN    BODY 


but  a  few  windings,  the  change  taking  place  within  a  few 
weeks  or  even  days  in  correlation  with  a  change  of  food  (Fig. 
73).     Allowing  for  a  few  isolated  cases,  however,  the  canal, 
of  fishes  and  amphibians  is  short  and  becomes  considerably 
lengthened  in  the  Sauropsida  and  Mammalia,  where  the  in- 


FIG.   73.     Comparison  of  alimentary  canal   in    (A),  tadpole  and    (B), 
adult  frog.     [After  the  LEUCKART  wall  charts.] 

testine,  the  part  mainly  involved  in  the  increase  of  length, 
becomes  disposed  in  complicated  folds  and  windings.  These, 
although  apparently  wholly  irregular  in  their  disposition,  may 
be  referred  to  a  definite  system,  as  may  be  made  clear  by  a 


THE    DIGESTIVE    AND   RESPIRATORY    SYSTEM    ,265 

study  of  the  various  steps  in  the  process.  In  a  simple,  straight 
intestine,  the  starting  point  of  all  forms,  the  tube  is  enwrapped 
by  the  peritoneum  (serosa),  which  becomes  reflected  along 
the  mid-dorsal  line,  the  two  layers  becoming  applied  to  one 
another  to  form  a  suspensory  ligament,  the  mesentery,  which 
in  turn  is  attached  along  the  medial  line  of  the  body  wall 
ventral  to  the  vertebral  column. 

As  the  tube  elongates  it  lengthens  the  free  edge  of  this  mes- 
entery, the  effect  of  which  is  to  throw  it  into  sinuous  curves 
directed  alternately  to  right  and  left,  which  in  extreme  cases 
fall  from  side  to  side  in  the  form  of  long  and  narrow  loops. 
The  intestinal  windings  are  never  seen  in  this  typical  form, 
however,  owing  to  the  peristaltic  movements  of  the  museulosa 
which  cause  the  folds  and  loops  to  constantly  change  their  po- 
sition so  that  their  disposition  in  an  animal  is  never  the  same 
at  two  intervals  of  time.*  ^ 

The  subdivisions  into  which  the  canal  is  divided  anatomi- 
cally for  descriptive  purposes  depend  upon  localized  enlarge- 
ment or  constrictions,  the  formation  of  diverticula,  or  the 
presence  of  definite  digestive  glands,  and  become  more  definite 
and  numerous  in  higher  forms  as  these  features  gradually 
appear  and  become  more  emphasized.  The  first  portions  to 
differentiate  are  the  pharynx  and  stomach,  the  former  being 
a  funnel-shaped  enlargement  of  the  anterior  end,  characterized 
by  pairs  of  lateral  diverticula,  the  pharyngeal  pouches,  which 
may  break  through  to  the  exterior  and  form  slits;  the  latter 
a  spindle-shaped  or  sac-shaped  compartment  for  the  reception 
of  food  of  all  sorts  in  about  the  condition  in  which  it  is  swal- 
lowed. The  narro\ved  portion  between  these  forms  the  oesopha- 
gus. At  its  lower  end  the  stomach  is  bounded  by  a  restricted 
portion,  the  pylorus,  which,  by  a  specialization  of  the  circular 

*  This  constant  change  of  appearance  and  endless  variety  in  arrangement 
is  exactly  suited  to  the  demands  of  divination,  which  always  depends 
upon  a  large  amount  of  chance  variation  of  some  object;  and  it  is  very 
probable  that  the  Roman  augurs,  who  manufactured  prophecies  from  the 
inspection  of  the  entrails  of  the  sacrificial  animals,  were  possessed  of 
as  definite  a  system  as  is  seen  to-day  in  the  case  of  palmistry,  a  "  science  " 
founded  like  the  other  upon  the  erroneous  and  utterly  baseless  assumption 
of  the  causal  relation  of  two  unrelated  sets  of  phenomena. 


, 
V/ 

x 


266  HISTORY   OF    THE    HUMAN    BODY 

muscles,  forms  a  valve  capable  of  closing,  thus  converting  the 
stomach  temporarily  into  a  closed  sac. 

The  remainder  of  the  canal  may  be  comprehendingly  desig- 
nated intestine,  the  regional  differentiation  of  which  does  not 
appear  as  early  and  is  never  so  marked  as  in  the  anterior 
portion.  A  little  below  the  pylorus  it  develops  from  its  mucosa 
two  enormous  glands  or  gland-complexes,  liver  and  pancreas, 
which  grow  out  far  beyond  the  limits  of  the  walls  of  the  canal, 
retaining  their  connection  with  their  place  of  origin  through 
ducts.  The  portion  of  the  intestine  between  the  pylorus  and 
the  orifices  of  these  ducts  receives  the  special  name  of  duode- 
num. A  second  early  and  better  marked  subdivision  of  the  in- 
testine is  a  terminal  enlargement  which  generally  receives  the 
openings  of  the  urinary  and  reproductive  systems,  and  is  hence 
termed  the  cloacal  chamber,  or  simply  cloaca.  This  portion 
retains  its  importance  in  fishes,  amphibians  and  the  Sauropsida, 
but  in  mammals  it  plays  a  subordinate  role,  appearing  as  a 
distinct  organ  in  the  lowest  forms  alone,  the  monotremes. 

In  certain  definite  portions  of  its  length  the  alimentary  canal 
shows  a  tendency  to  throw  out  diverticula,  sac-like  or  tubular 
in  shape  and  designed  apparently  to  increase  the  general  sur- 
face. Of  those  the  most  important  are  the  lateral  pharyngeal 
pouches  previously  mentioned;  the  pyloric  cceca,  found  in 
fishes,  and  often  very  numerous;  colic  cceca,  at  the  beginning 
of  the  large  intestine  in  mammals  ;  and  cloacal  caeca,  found  in 
birds. 

Following  this  general  sketch  of  the  alimentary  canal,  its 
development  and  its  differentiation,  the  separate  portions  may 
be  considered  in  greater  detail  [Cf.  Fig.  72].  The  most  an- 
terior of  these  are  the  mouth  cavity  and  pharynx,  usually 
fused  into  one,  the  stomato-pharyngeal  cavity,  although  in 
mammals  the  development  of  the  soft  palate  forms  an  incom- 
plete separation  between  the  two.  This  cavity  opens  to  the 
exterior  through  the  mouth  opening  or  stoma,  which  appears 
to  be  of  two  types  in  accordance  with  its  surroundings  and 
equipment. 

The  first  of  these,  the  cydostoma,  is  that  seen  in  the  lam- 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     267 

prey  and  allied  forms,  which,  on  account  of  it,  are  termed  the 
Cyclostomata.  This  type  is  similar  to  that  of  Amphioxus 
and  may  be  related  to  it;  it  is  circular  in  shape  and  equipped 
with  horny,  epidermic  teeth. 

The  second,  or  gnathostoma,  is  furnished  with  a  movable ' 
pair  of  skeletal  jaws,  equipped  with  teeth  formed  of  dentine 
overlaid  with  enamel.  In  origin  these  jaws  are  a  pair  of  vis- 
ceral arches  and  the  teeth  are  locally  modified  placoid  scales 
(See  Chap.  III.),  and  it  may  be  held  either  that  the  gnatho- 
stoma or  jaw-mouth  is  the  same  as  the  first,  to  which  the 
gill-arches  with  their  associated  teeth  have  become  added;  or 
that  it  is  a  new  opening,  originally  a  pair  of  gill-slits,  which 
have  become  fused  in  the  mid-ventral  line,  and  that  the  first 
mouth  has  become  lost.  In  favor  of  this  latter  view  is  the 
position  of  the  gnathostome  in  the  selachians,  where  it  may 
be  expected  to  show  the  most  primitive  condition,  and  where 
it  is  not  at  the  anterior  end  but  on  the  ventral  side  with  a 
long  rostrum  anterior  to  it.  Later  on,  as  is  shown  in  some 
ganoids,  it  attains  secondarily  an  anterior  terminal  position. 

If  from  the  evidence  presented  an  hypothetical  sketch  be  per- 
missible it  may  be  allowed  that  in  early  vertebrates  there  was 
a  circular  jawless  mouth,  provided  with  a  hood  and  situated 
at  the  anterior  end  of  the  body;  that  in  some  form  midway 
between  the  lamprey  eel  and  the  shark  the  habit  arose  of 
seizing  and  taking  in  food  by  the  anterior  gill-slits,  the  edges 
of  which,  provided  with  sharp,  pointed  scales,  served  better 
for  the  retention  of  their  living  prey  than  did  the  oral  hood 
and  horny  teeth  of  the  actual  mouth.  The  continuance  of  this 
habit  would  perfect  the  tools  employed,  which  in  this  case  were 
movable  gill-arches  armed  with  placoid  scales,  and  the  new 
mouth,  formed  by  the  ventral  fusion  of  two  lateral  slits  and 
furnished  with  superior  organs  of  prehension,  entirely  usurped 
the  function  of  the  old  one,  which  thus  became  reduced  and 
finally  disappeared.  In  one  point  alone,  that  of  position,  was 
the  old  mouth  superior,  and  the  final  step  in  the  perfection  of 
the  new  one  was  its  gradual  migration  to  the  anterior  end, 
the  various  steps  in  the  attainment  of  which  may  be  seen  among 
the  ganoids. 


268 


HISTORY    OF    THE    HUMAN    BODY 


Behind  the  mouth  on  each  side  there  develops  a  row  of  out- 
pushings,  the  pharyngeal  pockets,  which  meet  a  corresponding 
set  of  inpushings  from  the  outside  (Fig.  74).  In  fishes  these 
break  through  at  the  points  of  contact,  and  form  the  gill-slits, 
a  series  of  permanent  openings  (4-8  in  number)  that  form  a 
communication  between  pharynx  and  exterior  and  allow  the 


Fio.  74.     Pharynx  and  visceral  arches  in  human  embryo.     [After  His.] 

(a)  Ventral  aspect  of  early  embryo,  with  the  front  part  of  the  lower  jaw  and  the 
gill  arches  removed,  (b)  Inner  view,  looking  ventrally,  of  the  lower  jaw  and  gill- 
arches,  corresponding  to  the  part  removed  from  (a),  but  taken  from  a  somewhat 
younger  embryo.  (c)  Similar  to  (b),  but  taken  from  an  older  embryo,  not  far 
from  the  age  of  (a). 

In  (a)  the  most  anterior  of  the  arches  sectioned  is  the  mandibular  (  =  lower  jaw), 
succeeding  which  are  the  gill-arches  in  order,  three  being  definitely  formed.  An- 
terior to  the  sectioned  arches  are  seen  the  two  superior  maxillary  processes,  which, 
by  their  later  union  form  the  upper  jaw.  Between  and  a  little  above  these  is  the 
fronto-nasal  process.  In  (b)  and  (c)  the  most  anterior  arch  is  the  mandibular,  with 
the  gill-arches  succeeding  it  in  order.  The  round  median  mass  is  the  tuberculum  im- 
par,  which,  with  the  eminences  immediately  behind  it,  form  the  tongue.  The  thy- 
reoid  anlage  and  the  glottis  and  epiglottis  are  seen  in  (c). 


escape  of  the  water  constantly  taken  in  at  the  mouth  and 
used  in  respiration.  One  or  more  of  these  slits  appear  in 
amphibian  larvae  and  in  a  few  forms  persist  throughout  life, 
but  in  reptiles,  birds,  and  mammals,  although  the  pharyngeal 
pockets  and  their  corresponding  external  depressions  form 
during  embryonic  life,  but  two  or  three  ever  break  through  and 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     269 

then  for  a  very  short  period.  The  most  anterior  of  these, 
however,  which,  in  selachians,  appears  as  the  spiraculum,  or 
blow-hole,  persists  in  all  higher  vertebrates,  as  the  Eustachian 
tube  [tuba  auditiva  BNA~\  and  the  middle  ear.  The  remain- 
ing pockets  disappear  as  such,  but  various  accessory  structures, 
such  as  cartilages,  muscles,  arteries,  and  glands,  arise  in  the 
embryo  in  association  with  them  and  afterwards  become  modi- 
fied to  subserve  numerous  important  purposes.*  A  not  uncom- 
mon malformation  in  man,  a  few  cases  of  which  have  been 
reported  in  other  mammals,  is  that  of  a  cervical  fistula,  which 
forms  an  open  communication  between  pharynx  and  exterior, 
usually  upon  one  side  alone.  This  is  nothing  more  or  less 
than  a  permanent  gill-slit  and  may  be  considered  as  a  case  of 
arrested  development,  or  the  retention  of  what  is  designed  to 
be  a  transitory  stage. 

The  nasal  cavities,  which  lie  above  the  anterior  part  of 
the  stomato-pharyngeal  cavity,  are  in  fishes  quite  independent 
of  the  latter,  but  come  into  direct  communication  with  it  in 
Amphibia  by  the  formation  of  a  pair  of  openings,  the  posterior 
nares  or  choance,  which  appear  in  the  roof  of  the  mouth.  This 
communication  was  apparently  one  of  the  changes  inaugurated 
during  the  transition  from  water  to  land,  and  allows  the  in- 
gress and  egress  of  air  to  the  pharynx  and  thence  to  the  lungs 
without  opening  the  mouth,  since  this  action,  although  harm- 
less for  an  animal  immersed  in  water,  would  soon  cause  the 
drying  up  of  the  mucous  membrane  lining  the  mouth  cavity 
if  resorted  to  in  air  with  anywhere  near  the  same  frequency. 
In  the  case  of  the  nasal  cavities  this  is  prevented  in  part  by  the 
small  size  of  the  external  openings,  but  still  more  by  the  for- 
mation of  slime  glands  capable  of  producing  an  abundant  se- 
cretion. The  waste  lacrimal  fluid  conveyed  from  the  eyes 
to  the  nose  is  undoubtedly  also  of  assistance  in  this  respect. 

The  posterior  part  of  the  pharynx  shows  a  strong  tendency 
to  form  median  diverticula,  either  dorsal  or  ventral,  which  ex- 

*  Cf.  Chap.  V  under  Visceral  skeleton ;  Chap.  VI  under  Visceral  mus- 
cles, Chap.  VIII  under  Arterial  arches;  and  the  present  chapter  farther 
on  under  Thymus  and  Thyreoid. 


270  HISTORY   OF   THE    HUMAN    BODY 

pand  into  large  sacs  or  reservoirs  and  either  retain  or  lose 
their  communication  with  the  parent  cavity.  Such  are  the 
various  sorts  of  air-bladders  found  among  fish ;  these  are  gen- 
erally situated  dorsally  with  respect  to  the  pharynx,  but  ven- 
tral ones  occur  in  a  few  ganoids.  Occasionally  these  reser- 
voirs possess  an  attenuated  pneumatic  duct,  communicating 
with  the  pharynx,  and  the  supply  of  air  is  regulated  by  the 
fish  coming  to  the  surface  and  making  a  snapping  or  swallow- 
ing movement ;  but  in  the  majority  of  cases  the  air-bladder 
is  a  closed  sac,  filled  by  gases  extracted  from  the  blood.  In 
terrestrial  vertebrates  there  appears  in  the  embryo  a  mid- 
ventral  diverticulum  which  opens  from  the  floor  of  the 
pharynx,  grows  posteriorly  and  forks  into  two  lateral  branches. 
This  forms  the  pulmonary  system,  the  lateral  sacs  becoming 
lungs  and  bronchi,  the  median  duct  the  trachea,  and  the  open- 
ing in  the  pharynx  the  glottis,  which  becomes  regulated  by  a 
series  of  cartilages  and  muscles  derived  from  the  visceral  sys- 
tem and  forming  the  larynx. 

/""The  idea  naturally  suggests  itself  that  this  ventral  pulmon- 
/ary  system  is  a  direct  inheritance  from  a  similarly  situated 
/  air-bladder,  such  as  actually  occurs  in  some  ganoids,  and  al- 
/  though  there  is  no  direct  proof  of  this,  it  seems  very  probable. 
It  has  also  been  noted  that  the  diverticulum  which  produces 
it  lies  immediately  back  of  the  converging  line  of  pharyngeal 
pockets  and  it  has  thus  been  interpreted  by  some  as  a  continua- 
tion of  the  system,  the  forking  into  the  two  lungs  being  taken 
as  proof  of  the  formation  of  the  median  diverticulum  from  the 
confluence  of  two  lateral  pockets.  The  first  of  these  theories 
seems  by  far  the  more  probable,  especially  since  the  air-blad- 
der of  certain  ganoids  is  richly  supplied  with  respiratory 
blood-vessels,  and  thus  forms  a  better  lung  physiologically 
than  that  of  the  more  primitive  amphibians,  which  are  often 
simple  sacs,  yet  a  direct  continuity  from  one  to  the  other  can- 
not be  traced,  since  the  lungs  are  always  paired  and  an  air- 
bladder  is  always  single. 

The  nasal  cavities  are  separated  from  the  pharynx  by  an 
approximately  flat  plate  of  bone  that  forms  the  roof  of  the 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     271 

mouth,  or  hard  palate.  In  fishes  and  amphibians  this  is  formed 
in  great  part  by  the  anterior  end  of  a  single  extensive  median 
bone,  the  parabasal,  but  in  higher  forms  this  element  is  reduced 
and  its  function  is  assumed  by  the  palatines  and  pterygoids 
and  by  horizontal  plates  directed  inward  from  the  maxillaries 
and  premaxillaries.  These  latter  elements  are  all  lateral  and 
arise  in  the  embryo  upon  the  sides  of  the  head  and  grow  to- 
wards the  center,  finally  uniting  in  the  median  line.  In  some 
groups  of  birds  the  two  halves  remain  disassociated,  thus 
forming  a  palatine  cleft  by  which  a  direct  communication  is 
established  between  the  nasal  and  pharyngeal  cavities.  This 
failure  to  unite  may  occur  as  an  abnormality  in  mammals,  pro- 
ducing the  malformations  known  as  hare-lip  and  cleft  palate, 
malformations  thus  attributable  to  the  principle  of  arrested 
development.  In  certain  mammals,  as  the  cat  and  dog,  the 
remains  of  the  closure  are  permanently  shown  in  the  form  of 
a  deep  median  groove,  the  philthrum,  which  partly  divides  the 
upper  lip  and  runs  along  the  septum  of  the  nose  externally. 

In  mammals  the  bony  palate  (hard  palate),  which  is  con- 
posed  of  horizontal  processes  from  premaxillaries  and  maxil- 
laries and  a  portion  of  the  palatines,  is  continued  posteriorly 
into  a  membranous  soft  palate  or  velum  palati.  This  is  a  dupli- 
cation of  the  pharyngeal  mucous  membrane  and  is  supplied 
with  semi-voluntary  muscular  fibers  from  the  sheets  surround- 
ing the  pharynx,  which  are  themselves  probably  derivatives 
of  the  musculosa  of  the  anterior  portion  of  the  alimentary 
canaL 

Accompanying  the  stomato-pharyngeal  division  of  the  ali- 
mentary canal  are  several  important  auxiliary  organs  derived 
from  various  sources.  These  are  the  teeth,  tongue,  tonsils,  the 
glands  of  the  mouth  cavity,  and  the  glands  of  the  pharyngeal 
pockets,  which  will  be  considered  in  the  order  given. 

Excepting  the  horny  formation  in  the  mouth  of  the  Cyclo- 
stomata,  which  are  isolated  structures  of  epidermic  origin, 
the  teeth  of  the  vertebrates  are  strictly  homologous  organs 
throughout.  They  were  originally  placoid  scales  which  dif- 
ered  in  no  respect  from  those  that  cover  the  exterior  of  the 


272  HISTORY   OF   THE    HUMAN    BODY 

present-day  selachians,  and  have  been  modified  in  form  through 
the  change  of  function  due  to  their  position  in  and  about  the 
mouth  cavity.  Teeth  and  placoid  scales  correspond  closely  in 
structure  and  development,  and  consist  of  a  basis  of  dentine, 
or  "  ivory/'  as  it  is  often  called,  overlaid  by  enamel,  the  first 
being  formed  from  the  corium,  the  latter  from  the  epidermis. 

r  In  their  original  distribution  as  seen  in  fishes  and  amphibi- 
ans they  occur  not  only  along  the  edges  of  the  jaw,  but  also 
in  patches  over  the  roof  and  floor  of  the  mouth  cavity,  co- 
extensive with  the  stomatodseum ;  but  in  most  reptiles  and  in 

•  mammals  they  are  confined  to  a  single  row  in  each  jaw.  Cor- 
responding to  their  origin  the  most  primitive  arrangement  is 
that  of  an  imbricated  pattern  as  in  other  scales,  a  condition 
shown  in  many  of  the  areas  within  the  mouth  cavity,  and  in 
the  jaw  teeth  of  many  selachians,  where  they  appear  in  several 
rows.  Primarily,  at  the  stage  in  which  the  jaws  and  skull 
are  cartilaginous,  as  in  modern  selachians,  the  teeth  are 
separated  from  one  another,  but  in  a  slightly  higher  stage  the 
bases  fuse  for  mutual  support,  thus  forming  a  flat  plate  of 
bone  upon  which  the  separate  tooth  elements  appear  as  pro- 
jecting points  or  cusps.  This  proceeding  is  repeated  ontoge- 
netically  in  a  few  cases,  as  in  the  paired  vomers  of  the  frog; 
although  usually  by  a  shortening  of  the  development  the  stage 
at  which  the  scales  are  separate  is  dropped  out  and  the  bony 
plate  develops  as  a  single  structure,  so  that  in  cases  where  the 
cusps  have  become  lost  there  is  no  indication  of  their  dental 
origin.  Thus  are  formed  the  Hat  bones  that  line  the  mouth 
cavity,  such  as  the  vomers,  palatines,  pterygoids,  and  parabasal, 
which  in  lower  forms  often  retain  their  dentigerous  character; 
such  is  also  the  origin  of  the  premaxillaries  and  maxillaries 
that  form  the  upper  jaws,  as  well  as  that  of  the  splint-like 
dentare,  and  perhaps  the  angulare,  that  enclose  Meckel's  carti- 
lage and  form  the  mandible.  (See  Chap.  V.)  The  cusps  are 
thus  originally  an  integral  part  of  the  bony  splints  which  bear 
them,  and  in  fishes,  amphibians,  and  most  reptiles  the  two 
remain  continuous,  but  in  crocodiles  and  mammals,  as  well 
as  in  the  fossil  toothed  birds,  the  two  become  detached,  and  the 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     273 

teeth  are  inserted  by  what  is  termed  a  thecodont  articulation 
into  deep  pits  in  the  bone  called  thccce  or  alveoli  (Fig.  75). 

That  portion  of  the  tooth  which  fits  into  an  alveolus  is 
termed  the  root  and  is  covered  by  a  sort  of  bone  called  cement, 
while  that  which  appears  above  the  gum  is  called  the  crown, 
and  is  the  only  part  overlaid  by  enamel. 

All  teeth  are  hollow  and  contain  a  central  pulp  cavity,  which 
encloses  a  nutrient  corium  papilla;  in  cases  where  the  cusp  is  a 
part  of  the  bone  the  pulp  cavity  is  open  along  one  side  to 
admit  the  passage  of  nerves  and  blood-vessels.  In  thecodont 
teeth,  however,  the  latter  come  up  through  the  bottom  of  the 
theca  and  enter  the  pulp  cavity  at  the  inner  end  of  the  root. 


a  b  c  d 

FIG.  75.     Types  of  teeth. 

(a),    pleurodont;     (b),    acrodont;     (c)     thecodont    with    open    root;     (d)     thecodont 
with  closed  root. 

In  this  respect  there  are  two  kinds  of  roots,  open  "and  closed, 
in  the  first  of  which  the  root  is  widely  opened  and  not  dis- 
tinct in  structure  from  the  crown,  while  in  the  latter  the 
root  is  nearly  solid  and  its  lumen  is  restricted  to  a  fine  canal 
through  which  the  vessels  may  reach  the  pulp.  In  both  cases 
layers  of  new  dentine  are  constantly,  though  slowly,  added  to 
the  rest  through  the  agency  of  a  layer  of  cells  called  odonto- 
blasts,  which  cover  the  pulp  and  are  firmly  applied  to  the  inner 
walls  of  the  pulp  cavity.  In  the  first  type,  in  which  the  root 
is  widely  open  and  the  entire  tooth  fits  like  a  cap  over  the 
pulp,  the  tooth  is  gradually  pushed  upward  by  the  addition 
of  new  layers  underneath,  and  is  thus  continually  elongating, 
while  in  the  second  or  closed  type  the  addition  of  new  layers 


274  HISTORY    OF    THE    HUMAN    BODY 

merely  diminishes  the  size  of  the  pulp  cavity  without  increas- 
ing the  length  of  the  tooth  as  a  whole.  Both  methods  are  in 
a  way  a  provision  against  the  loss  of  substance  due  to  constant 
use ;  in  the  first  type  the  outward  growth  and  the  loss  through 
wear  usually  balance  one  another  so  that  the  tooth  remains 
of  about  the  same  length  throughout  life;  in  the  second 
the  tooth  grows  gradually  shorter  while  the  addition  of  new 
layers  beneath  merely  protects  the  sensitive  pulp  by  keeping 
a  constant  thickness  of  dentine  between  it  and  the  free  surface. 
Thus,  in  man,  whose  teeth  are  of  the  second  type,  the 
chewing  surface  in  old  age  is  at  a  level  which  in  youth  would 
lay  bare  the  pulp.  Illustrations  of  the  first  type  are  seen  in 
the  teeth  of  the  hippopotamus,  the  tusks  of  swine  and  ele- 
phants, and  the  chisel-like  incisors  of  rodents;  in  the  last  in- 
stance the  front  side  only  is  covered  with  enamel,  and  as  this 
substance  is  harder  than  dentine,  it  wears  away  more  slowly, 
and  constantly  presents  a  projecting  edge,  thus  keeping  the 
teeth  sharp.  Such  teeth  depend  upon  constant  use  in  order  to 
be  kept  at  the  proper  length,  and  in  abnormal  cases  in  which 
some  irregularity  of  the  jaw  prevents  the  meeting  of  opposite 
teeth,  they  grow  past  one  another  and  become  eventually 
disposed  in  coils  and  other  eccentric  forms  which  may  prove 
the  death  of  their  possessor  through  an  inability  to  feed  prop- 
erly. Similar,  although  here  perfectly  normal,  instances  are 
those  of  the  ornamental  tusks  of  elephants  and  wild  swine, 
and  in  one  of  these  latter,  the  babyroussa  of  the  East  Indies, 
an  upper  tooth  upon  each  side  bores  upward  through  the  lip 
and  erects  a  curved  point  high  above  the  snout. 
-  The  primitive  form  for  a  tooth  is  that  of  an  elongated 
cone,  a  type  which  occurs  with  slight  variation  in  proportions 
among  all  the  lower  vertebrates.  In  mammals,  however,  the 
teeth  are  characterized  by  the  introduction  of  numerous 
modifications,  sometimes  very  complex  in  nature,  resulting  in 
a  large  number  of  variations,  not  merely  in  different  species, 
but  in  different  regions  of  the  same  jaw,  which  correspond 
to  a  differentiation  in  use  between  the  front  teeth,  that  are 
in  a  position  for  grasping  the  food,  and  the  back  teeth,  which 


THE    DIGESTIVE   AND   RESPIRATORY   SYSTEM     275 

come  under  the  immediate  control  of  the  masticatory  muscles. 
In  the  Cetacea  alone  the  teeth  are  all  alike  (homodont)  and  of 
the  simple  conical  type,  and  here  it  may  be  considered  certain 
that  this  condition  is  a  secondary  one  and  that  the  teeth  have 
lost  the  usual  complexity  through  disuse;  since  an  aquatic 
animal  cannot  chew  and  employs  its  teeth  merely  for  the  pur- 
pose of  holding  and  retaining  the  food.  In  all  other  mammals 
the  teeth  are  heterodont,  and  possess  several  distinct  forms, 
which  have  been  conveniently  divided  into  three  types,  incisors, 
canines,  and  molars,  with  a  possible  subdivision  of  the  last 
into  premolars  and  molars  proper. 

Of  these  types  the  incisors  are  the  most  anterior  and  con- 
sist in  the  upper  jaw  of  those  borne  by  the  premaxillary  bones 
and  in  the  lower  jaw  of  those  opposite  the  former.  There  may 
be  five  of  these  in  each  upper  half  jaw,  and  four  in  each  lower 
half,  but  the  usual  number  is  three  or  two.  Corresponding 
to  the  most  frequent  function  of  teeth  thus  situated  their 
typical  shape  is  that  of  chisels  for  cutting  or  biting, 
a  form  easily  derived  from  the  primitive  conical  forms  by 
a  flattening  in  a  labio-lingual  direction.  Beyond  these 
there  is  in  each  half  jaw  a  single  canine,  typically  in  the 
form  of  a  pointed  cusp  elongated  beyond  the  level  of  the 
other  teeth  and  best  preserving  the  primitive  shape.  The  can- 
ines of  the  lower  jaw  lie  anterior  to  those  of  the  upper  jaw  and 
in  the  case  of  elongated  canines  slip  past  one  another.  As 
their  chief  policy  is  that  of  piercing  and  tearing,  they  become 
reduced  or  are  entirely  wanting  in  animals  in  which  this  func- 
tion is  superfluous.  The  remaining  teeth  may  be  classed  to- 
gether as  molars,  or  cheek-teeth,  or  a  distinction  may  be 
made  in  most  cases  between  an  anterior  group  of  premolars, 
in  which  the  first  teeth  are  usually  replaced  by  a  second  set, 
and  a  posterior  group  of  true  molars,  which  develop  later  than 
the  rest  and  are  not  replaced.  That  this  distinction  is  in  part 
an  artificial  one  and  often  difficult  or  impossible  of  application 
is  shown  by  the  study  of  the  subject  of  replacement,  consid- 
ered below.  Of  premolars  the  greatest  number  that  occurs 
in  mammals  is  four  in  each  half-jaw,  of  definite  molars,  five. 


276  HISTORY   OF   THE    HUMAN    BODY 

The  number  of  each  kind  of  teeth  occurring  in  a  given 
mammal  is  briefly  and  graphically  expressed  by  a  dental  for- 
mula, which  may  be  written  in  a  variety  of  ways.  Thus  the 
entire  dentition  may  be  expressed  as  in  the  following  formula 
for  the  cat,  in  which  the  upper  and  lower  rows  represent  the 
two  jaws,  the  mid-ventral  line  is  marked  by  the  short  per- 
pendicular and  the  number  of  each  group  of  teeth  is  desig- 
nated by  a  digit: 

i-Vi-3 


1-2-1-3 


3-1-2-1 


This  formula  shows  at  once  that  there  are  in  each  jaw  six 
incisors  flanked  on  each  side  by  a  canine,  and  that  there  are 
four  cheek  teeth  above  and  three  below  on  each  side,  only  the 
last  one  of  which  is  not  replaced  and  is  therefore  a  true  molar. 

Except  for  the  sake  of  symmetry,  however,  one  half  alone 
may  be  given,  as  in  the  following  for  the  Bovidae,  the  family 
to  which  cattle  and  sheep  belong,  a  formula  showing  a  total 
loss  of  canines  and  of  upper  incisors : 

003-3 


3-03-3 

From  the  study  of  the  dentition  in  all  mammals  and  espe- 
cially that  of  marsupials  and  the  more  primitive  placental 
mammals,  the  following  hypothetical  dentition  has  been  de- 
duced for  the  ancestral  type,  to  which  all  existing  dentitions 
may  be  referred: 

5-4-T-5 


5-4-1-5 


5-1-4-5 


This  formula  provides  for  ten  incisors  and  eighteen  cheek 
teeth  in  each  jaw,  with  a  total  of  60  teeth,  30  in  each  jaw. 
This  formula  is  nearly  attained  by  some  marsupials,  where  the 
most  primitive  condition  would  be  expected,  but  in  placental 
mammals  there  is  always  a  reduction  of  incisors  and  molars, 
the  former  being  never  more  than  three  upon  each  side.  Thus, 
in  the  opossum  (a  marsupial)  the  dental  formula  is: 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     277 

the  last  figure  representing  the  total  number  of  cheek  teeth, 
without  attempting  a  distinction  between  premolars  and  mo- 
lars. Among  placental  mammals  the  Insectivora  show  the 
most  primitive  dentition;  for  example,  that  of  Talpa  (the 
mole)  is: 

3-1-4-3 


3-1-4-3 

Certain  cases,  on  the  other  hand,  show  an  extraordinary  re- 
duction, as  in  the  mouse-like  rodent,  Hydromys,  where  the 
formula  is : 

1-0-02 


1-O-O2 


The  greatest  reduction  is  found  among  some  Cetacea,  and  in 
certain  edentates  (Myrmecophaga).  In  certain  of  the  first 
group  tfre  tooth  germs  never  develop  but  remain  within  the 
gums,  while  feeding  is  carried  on  through  the  development  of 
horny  fringes  (the  so-called  "whale-bone")  depending  from 
the  upper  jaw,  and  forming  a  filter,  which  transforms  the 
mouth  into  a  scoop-net  for  the  apprehension  of  shoals  of 
marine  creatures  of  various  sorts.  In  the  duck  whale,  Hy- 
per odd  on,  there  are  but  four  teeth,  all  in  the  lower  jaw,  the 
anterior  ones  of  conical  shape,  behind  which  are  two  imbed- 
ded in  the  gums;  in  the  narwhal,  Monodon,  there  are  numer- 
ous small  teeth  that  fall  out  before  .maturity,  leaving  the  jaw 
toothless,  but  in  the  male  a  single  upper  tooth,  usually  that 
of  the  left  side,  develops  into  a  long,  projecting  horn  so  large 
that  it  renders  the  skull  asymmetrical.  In  certain  of  the 
smaller  Cetacea,  dolphins  and  porpoises,  the  jaws  are  fur- 
nished with  conical  teeth,  nearly  or  quite  homodont,  which 
often  surpass  in  number  those  of  any  other  mammals  (in  the 
Dolphin,  Delphinus,  47-65  in  each  half-jaw).  Both  this  ex- 
cessive number,  which  is  at  variance  with  the  general  formula 
for  mammalian  dentition,  and  their  homodont  character,  must 
be  looked  upon  as  secondary  adaptations  to  aquatic  conditions. 
In  the  primates  there  are  two  incisors  upon  each  side  and 
a  well-developed  canine.  In  the  monkeys  of  the  Western 


278  HISTORY    OF    THE    HUMAN    BODY 

Hemisphere  (Platyrrhini)  there  are  three  premolars  and  either 
two  or  three  molars,  but  in  those  of  the  Eastern  Hemisphere 
(Catarrhini)  there  are  always  two  of  the  former  and  three  of 
the  latter,  giving  the  constant  formula  of: 

2-I-2-1* 

=  V2 

2-1-2-3  6 

Of  these  the  five  medial  teeth  of  each  jaw  are  replaced  by  a 
second  set,  the  three  molars  not,  the  formula  for  the  milk 
dentition  being: 

2-1-2-0 


2-1-2-0 


20 


In  these  points  man  corresponds  completely  with  the  other 
Catarrhini. 

Regarding  the  evolution  of  the  various  shapes  of  mammalian 
teeth  from  the  primitive  conical  type,  the  canines  and  incisors 
present  a  simple  problem,  since  the  first  retain  .almost  their 
typical  form,  and  the  second  show  merely  a  labio-lingual 
flattening.  The  derivation  of  the  complex  cheek  teeth,  how- 
ever, presents  a  serious  problem,  for  the  solution  of  which  two 
main  theories  have  been  offered.  According  to  the  first,  or 
tritubercular,  theory,  the  fundamental  postulate  must  be  laid 
down  that  every  tooth.,  no  matter  how  complex,  represents  a 
single  primary  element,  and  that  the  modifications  are  due 
to  the  development  of  additional  cusps  for  the  purpose  of  in- 
suring a  better  articulation  with  the  opposing  surfaces  of  the 
teeth  of  the  other  jaw.  The  number  of  cusps  which  may  thus 
develop  on  the  contact  surface  of  a  tooth  is  si.vf  the  first  of 
which  to  appear  is  the  protocone,  or  primary  cusp.  Associated 
with  the  protocone  are  two  secondary  ones,  the  paraconef 
which  is  anterior  to  it,  and  the  metacone,  which  is  posterior 
(Fig.  76,  I  and  II).  These  may  become  connected  by  crests 
or  ridges  and  the  tooth  may  become  still  more  complicated 
by  the  bending  of  the  crests  and  cusps  into  a  V-shaped  figure, 
the  trigon  (Fig.  76,  III).  A  tooth,  or  a  dentition,  in  which 
the  three  cusps,  para-,  proto-,  and  meta-cone,  still  lie  in  a 


THE    DIGESTIVE    AND    RESPIRATORY    SYSTEM     279 


straight  line  following 
the  line  of  the  jaw, 
is  called  triconodont, 
one  in  which  the  bend- 
ing has  taken  place  is 
trigonodont.  This  tri- 
tubercular  tooth,  still 
under  the  influences  of 
the  opposing  tooth  sur- 
faces, may  develop  a 
lateral  spur,  the  hypo- 
cone,  upon  which  1-3 
tertiary  cups,  the  co- 
nidi,  may  develop,  or 
else  the  hypocone  may 
form  the  point  of  a  pro- 
jecting spur,  the  talon. 
The  nomenclature  given 
here  is  that  of  the  up- 
per teeth,  the  corre- 
sponding parts  of  the 
lower  jaw  being  distin- 
guished by  the  addition 
of  the  suffix  -id,  thus : 
protoconid,  hypoconid, 
talonid,  etc.  In  favor 
of  this  theory  may  be 
urged  its  complete  ap- 
plicability to  all  known 
forms,  both  living  and 
fossil,  as  a  system  of 
nomenclature,  and  its 
correspondence  in  se- 
quence of  stages  to  that 
shown  by  the  extinct 
forms  in  consecutive 
geological  periods.  The 


rt 


FIG.  76.  Phyletic  history  of  the  molar 
cusps.  [After  H.  E.  OSBORN.] 

I.  Reptilian  stage,  haplodont;  Permian.  II. 
Triconodont  stage.  III.  Tritubercular  stage. 
IV.  Tritubercular-tuberculo-sectorial ;  lower  Ju- 
rassic. V.  The  same,  upper  Jurassic.  VI.  The 
same,  upper  Cretaceous.  VII.  The  same,  Puerco, 
lower  Eocene.  VIII.  Sexitubercular-sexitubercu- 
lar,  Puerco.  IX.  Human  lower  molar. 

Pt,  protocone;  P,  paracone;  m,  metacone;  h 
hypocone;  Ptd,  protoconid;  Pd,  paraconid;  mi, 
metaconid;  hd,  hypoconid;  hid,  hypoconulid;  ed, 
entoconid. 


280  HISTORY    OF    THE    HUMAN    BODY 

embryological  record,  also,  with  a  few  exceptions,  which  may 
be  explained  in  other  ways,  is  in  accord  with  it. 

A  second  theory  to  account  for  the  complex  form  of  molars 
is  that  they  are  in  reality,  as  they  are  often  termed,  "  double 
teeth/'  and  arise  from  the  fusion  of  several  primary  germs. 
This  concrescence  theory  is  much  older  than  the  first,  and  had 
been  generally  abandoned  in  favor  of  the  other  when  its 
probability  in  the  case  of  certain  forms  was  recently  reasserted, 
owing  to  the  testimony  of  embryology.  It  is  thus  possible 
that  both  theories  may  be  true  as  applied  to  different  cases, 
the  tritubercular  method  being  the  more  general. 

A  widespread  phenomenon  among  mammals  is  that  of  the 
replacement  at  a  definite  period  of  certain  of  the  anterior 
teeth  by  a  second  set,  and  in  respect  to  this  power  mammals 
are  classed  as  diphyodont,  in  which  such  a  replacement  oc- 
curs, and  monophyodont,  where  but  one  set  appears.  Al- 
though in  man  and  in  many  domestic  animals,  in  which  this 
procedure  was  first  studied,  the  distinction  between  the  two 
sets  is  clear  and  definite,  such  is.  not  the  case  among  certain 
of  the  mammals,  and  careful  embryological  records  which 
show  numerous  cases  of  rudimentary  tooth  germs  both  pre- 
ceding and  succeeding  definite  teeth,  has  caused  a  revision 
of  the  entire  subject.  The  matter  becomes  more  compre- 
hensible by  referring  to  the  lower  vertebrate  Classes,  espe- 
cially reptiles,  where  the  papilla  of  each  physiologically  active 
tooth  is  associated  with  a  succession  of  additional  tooth  germs 
in  different  stages  of  maturity,  and  designed  to  replace  the 
functional  tooth  in  case  of  injury  (Fig.  77). 

In  a  vitally  important  tooth,  as  in  the  poison  fangs  of  ser- 
pents, at  least  one  replacement  tooth,  nearly  ready  for  use, 
lies  continually  by  the  side  of  the  functional  one,  and  may 
develop  almost  at  a  moment's  notice  in  case  of  the  loss  of  the 
latter.  This  power,  limited  to  a  single  generation  of  replace- 
ment teeth,  restricted  to  that  part  of  the  jaws  anterior  to  the 
true  molars,  and  arranged  to  assert  itself  at  a  certain  definite 
period  in  the  case  of  each  tooth  would  result  in  the  two  den- 
titions of  the  typical  diphyodont  Mammalia  (Fig.  78,  A),  and 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     281 

such  has  been  undoubtedly  the  origin  of  this  phenomenon,  save 
that  it  is  not  necessary  to  assume  that  all  the  feeth  of  either  set 
belong  to  the  same  original  generation,  especially  as  they  do 
not  develop  simultaneously.  Thus  some  have  seen  in  the  suc- 
cession of  mammalian  teeth  the  remains  of  five  separate  tooth 
generations,  individuals  from  several  uniting  in  a  specific  case 
to  form  a  set,  either  the  milk  or  the  permanent  one.  Others, 
admitting  that  each  of  the  two  definite  sets  represents  a  single 
tooth  generation,  find  in  certain  cases  traces  of  tooth  germs 


FIG.  77.     Succession  of  teeth  in  reptiles. 

(A)  Section  through  jaw  of  Phyllodactylus  (a  lizard)  showing  functional  tooth 
(I)  and  several  replacement  teeth  (II,  III);  m,  Meckel's  cartilage;  n,  nerve.  [After 
GEGENBAUR.]  (B)  Diagram  of  integument  of  gum,  to  explain  the  succession  of  teeth. 
str.  cor.,  stratum  corneum;  str.  muc.t  stratum  mucosum;  /,  II,  etc.,  the  succession 
of  tooth  germs. 

that  precede  the  milk  dentition,  and  others  that  succeed  the 
permanent  set;  they  thus  consider  that  mammals  have  in- 
herited from  their  reptilian  ancestors  four  tooth  generations, 
prelacteal,  lacteal,  permanent  and  post-permanent,  of  which 
the  second  and  third  have  become  generally  established.*  The 
first  developed  in  the  premammalian  ancestors,  the  last  may 
come  to  development  in  the  future. 

Regarding  the   occurrence  of  the  two  dentitions  there   is 

*  It  is  asserted  that  in  Nasodon.  a  Tertiary  ungulate,  a  pair  of  prelacteal 
incisors  becomes  developed. 


282 


HISTORY   OF   THE   HUMAN   BODY 


much  variation.  In  certain  marsupials  but  one  tooth  is  re- 
placed, the  last  upper  premolar,  this  tooth  alone  constituting  a 
second  set,  while  otherwise  the  milk  set  is  retained  through  life. 


A 


B 


FIG.  78.     Figures  representing  diphyodont  dentition. 

(A)  Dentition  of  a  lion  cub  of  six  and  a  half  months.  [After  WEBER.]  Milk 
dentition  functional,  replacement  teeth  still  within  the  jaw  and  shown  as  shaded 
areas.  (B)  Upper  jaw  of  new-born  seal,  with  replacement  teeth  almost  ready  for 
function.  The  milk  dentition  is  reduced  to  useless  rudiments  cast  off  immediately 
after  birth.  [From  HERTWIG,  after  BURCKHARDT.] 

They  are  thus  almost  monophyodont,  with  a  permanent  milk 
dentition,  and  are  in  sharp  contrast  to  other  monophyodont 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     283 

mammals  in  which  the  milk  set  is  represented  by  useless  rudi- 
ments that  are  either  absorbed  before  birth  or  expelled  soon 
after.  In  view  of  all  facts  thus  far  presented  it  seems  that 
the  monophyodont  condition  has  been  secondarily  acquired  as 
a  special  adaptation  in  certain  forms  by  the  suppression  of 
one  or  the  other  of  the  two  typical  generations  (Fig.  78,  B). 
Although  fishes  possess  no  functional  tongue_ihe  material 
out  of  which  a  tongue  is  to  be  constructed  is  present  in  the 
form  of  the  anterior  part  of  the  hyo-branchial  apparatus. 
The  anterior  border  of  this  complex  lies  in  the  floor  of  the 
mouth  cavity,  following  the  outlines  of  the  jaw,  and  by  certain 
actions  of  the  visceral  muscles  may  be  projected  upwards  so 
as  to  form  a  noticeable  elevation.  When  in  amphibians  these 
parts  become  relieved  from  the  gill-bearing  function,  they 
form  the  basis  of  the  tongue,  often  fusing  into  a  complex, 
moved  by  muscles  and  supporting  a  fleshy  organ  of  some  sort; 
In  that  type  of  tongue  which  leads  to  the  higher  vertebrates 
the  skeletal  basis  consists  of  two  to  four  of  the  visceral  arches/ 
beginning  with  the  hyoid,  and,  although  admitting  of  many 
varieties,  possesses  as  essential  a  median  basi-branchial  piece, 
called  here  the  os  cntoglossum,  and  two  posteriorly  projecting 
cornua.  The  tongue  of  the  Sauropsida  is  a  direct  continuation 
of  this,  and  in  both  cases  the  principal  motion  is  a  protrusion 
and  withdrawal  of  the  organ  as  a  whole,  which  is  effected  by 
means  of  the  two  posterior  cornua,  which  lie  in  sheaths  from 
which  they  may  be  everted.  When  the  tongue  is  unusually 
long  the  sheaths  and  the  enclosed  cornua  are  of  correspond- 
ing length,  and  their  disposition  when  retracted  becomes  a 
problem  variously  solved  in  different  cases.  Thus  in  a  cer- 
tain salamander,  Spelerpes  fuscus,  the  sheaths  of  the  cornua 
run  down  the  sides  of  the  body  and  are  attached  to  the  ilia, 
and  in  the  woodpecker  they  come  around  the  occipital  region, 
pass  over  the  top  of  the  head  and  terminate  at  the  base  of  the 
upper  beak,  near  the  anterior  nares.  In  these  cases  the  ends 
of  the  cornua  are  attached  to  the  bottom  of  the  sheaths  and  as 
they  are  withdrawn  the  sheath  is  turned  inside  out  and  adds 
to  the  total  length. 


284  HISTORY   OF   THE    HUMAN    BODY 

In  mammals  the  hyo-branchial  support  of  the  tongue  is 
reduced  to  a  complex  composed  of  a  basi-hyal  (body)  and  two 
pairs  of  cornua,  of  which  the  anterior  are  typically  the  longer 
and  are  formed  by  a  chain  of  four  skeletal  pieces,  cerato-,  epi-, 
stylo-  .and  tympano-hyal,  the  latter  attached  to  the  tympanic 
region  of  the  skull.  The  posterior  cornua  consist  each  of  a 
single  piece,  thyreo-hyal,  connecting  the  body  with  the  thyre- 
oid  cartilage  of  the  larynx.  In  man  the  anterior  cornua  show 
an  unusual  modification,  the  tympano-  and  stylo-hyals  are 
fused  with  the  otic  region  of  the  skull,  forming  the  styloid 
process,  the  hypo-hyal  is  reduced  to  a  rudiment  which  con- 
nects with  the  styloid  process  by  a  ligament  in  which  no  trace 
of  the  cerato-hyal  is  seen.  Thus  the  anterior  cornua,  though 
typically  longer  and  more  complex  than  the  others,  are  spoken 
of  in  man  as  the  "  lesser,"  an  inheritance  from  the  earlier 
anatomical  science  in  which  comparison  with  other  mammals 
played  no  part. 

Inserted  upon  this  skeletal  complex  as  a  basis  there  is  de- 
veloped in  mammals  a  fleshy  tongue  composed  of  interlaced 
muscular  fibers,  a  part  of  which  are  intrinsic  and  belong  to 
the  tongue  itself,  while  others  are  extrinsic  and  consist  of  the 
terminal  fibers  of  other  muscles.  This  organ  is  thus  a  struc- 
ture totally  unlike  the  tongue  of  most  other  vertebrates,  in 
which  the  skeletal  support  reaches  through  the  organ  and  in 
which  motion  is  confined  to  a  simple  protrusion  and  retraction, 
and  resembles  rather  the  fleshy  lobe  which  appears  in  a  few 
cases  appended  to  the  other  structure,  as  in  the  frog.  More- 
over, in  some  mammals,  notably  marsupials  and  lemurs,  there 
exists,  beneath  the  fleshy  organ,  an  accessory  tongue-like  struc- 
ture, the  sub-lingua,  which  possesses  many  attributes  of  the 
tongue  of  the  Sauropsida  and  like  it  is  supported  by  a  car- 
tilaginous piece  which  may  represent  the  os  entoglossum.  In 
man  the  sub-lingua  is  reduced  to  a  transverse  fold,  the  plica 
timbriata.  readily  seen  if  the  mouth  be  opened  and  the  tongue 
elevated.  If  this  organ  be  taken  as  the  homologue  of  the 
sauropsidan  tongue,  as  its  structure  and  position  seem  to  indi- 
cate, the  fleshy  tongue  of  mammals  is  a  new  structure,  de- 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     285 

veloped  upon  the  dorsal  side  of  the  former,  and  gradually 
usurping  its  function.  An  opposing  opinion  rejects  the  claim 
of  the  sub-lingua  as  homolog  of  the  sauropsidan  tongue,  and 
finds  the  rudiment  of  the  os  entoglossum  in  the  septum  lingua, 
a  band  of  connective  tissue  running  lengthwise  through  the 
tongue  and  serving  as  an  attachment  for  the  muscles,  or  more 
definitely  in  the  lyssa,  a  vermiform  structure  composed  of 
connective  tissue,  fat,  cartilage,  and  muscle  fibers,  which  oc- 
curs in  the  tongue  of  certain  mammals,  and  is  often  asso- 
ciated with  the  septum. 

Distinctive  glands  in  the  mouth  cavity  other  than  simple 
mucous  cells  are  not  found  in  fishes  or  wholly  aquatic  am-i 
phibians  and  appear  only  as  an  adaptation  to  the  terrestrial/ 
life  for  the  primary  purpose  of  keeping  the  surface  moist.  \ 
Such  glands  are  thus  constantly  present  in  terrestrial  verte- 
brates and  become  secondarily  reduced  in  those  that  become 
readapted  to  the  water,  as  in  the  case  of  the  Cetacea.  These 
glands  are  often  voluminous,  occur  in  all  available  positions, 
and  are  named  accordingly,  buccal,  labial,  lingual,  sub-lingual, 
sub-maxillary  (more  properly  sub-mandibular) ,  etc.  Al- 
though the  primary  function  of  all  these  was  undoubtedly  that 
given  above,  they  have  in  numerous  instances  assumed  more 
special  functions  and  have  altered  the  chemical  nature  of  their 
secretion  accordingly.  Thus  the  intermaxillary  glands  of  frogs 
and  toads,  which  open  into  the  roof  of  the  mouth,  secrete  a 
viscid  fluid  by  means  of  which  the  tongue  is  rendered  adhesive 
for  the  apprehension  of  insects,  the  lingual  glands  of  many 
salamanders  and  lizards  have  a  similar  function,  and  certain 
buccal  glands  in  poisonous  serpents  become  the  elaborators  of 
the  venom,  which  is  inoculated  into  the  victim  by  means  of 
the  poison  fangs.  These  teeth  are  provided  either  with  a 
groove  along  the  external  surface  or  possess  a  minute  lumen 
through  the  center  as  in  the  case  of  a  hypodermic  needle. 
Another  secondary  function  extensively  employed  is  that  of 
lubricating  dry  food  to  render  it  more  easily  swallowed,  and 
in  association  with  this  the  secretion  often  develops  ferments, 
of  use  in  the  digestion  of  starch,  and  forms  the  saliva.  The 


286  HISTORY   OF    THE    HUMAN    BODY 

glands  specialized  to  produce  this  fluid  may  be  termed  salivary, 
without  reference  to  their  position,  and  consist  in  mammals 
of  the  parotid,  sub-mandibular  \_sub-maxillary  BNA~\,  sub- 
lingual,  and  retro -lingual,  the  last  associated  with  the  sub- 
mandibular  and  not  found  in  all  mammals.  There  also  occur 
other  voluminous  glands,  with  a  structure  similar  to  the 
others,  that  secrete  a  clear  fluid  without  salivary  attribute. 
Such  are  the  molar  glands  of  ungulates,  or  the  voluminous 
orbital  gjand  of  the  dog  family  (Canidae),  which  opens  by  a 
duct  into  the  mouth  cavity  in  the  region  of  the  last  upper 
molar. 

Associated  in  origin  with  the  pharyngeal  region  are  cer- 
tain organs  of  more  or  less  uncertain  significance  and  varied 
destiny,  the  most  prominent  of  which  are  the  thymus  and 
thyre oid  glands.  These,  although  not  connectecTTn  function 
with  the  dTgestive  system,  may  be  treated  here  because  of 
their  origin.  In  the  cyclostomes,  which  furnish  the  first,  or 
most  generalized  stage  in  this  history,  there  develop  seven 
pharyngeal  pockets  on  each  side,  each  with  a  dorsal  and  a 
ventral  recess.  About  each  of  these  there  develops  a  prolifera- 
tion of  epithelial  cells,  forming  an  organ  or  organ-anlage. 
These  anlagen  appear  alike  in  structure,  but  in  the  higher 
forms  the  differentiations  from  the  dorsal  series  become  col- 
lectively known  as  the  thymus,  those  from  the  ventral  as  epi- 
thelial corpuscles. 

The  later  history  of  the  thymus  anlagen  shows  three  ten- 
dencies, (i)  to  become  early  separated  from  their  layer  of 
origin,  (2)  to  fuse  upon  each  side  to  a  single  long  organ, 
often  showing  its  segmented  origin,  and  (3)  to  become  re- 
stricted in  number,  the  more  anterior  ones  being  the  first  to 
disappear.  These  are  all  seen  in  the  accompanying  diagram 
(Fig.  79),  which  shows  the  phylogenetic  steps  in  the  process. 
Thus,  beginning  with  the  seven  anlagen  of  the  cyclostomes, 
the  teleosts,  which  possess  six  pharyngeal  pockets,  show  but 
four,  and  these  the  most  posterior.  The  first  pocket  has  none, 
that  of  the  second  is  represented  by  a  rudiment,  which  early 
disappears.  In  the  urodeles,  in  which  five  pockets  appear, 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     287 


FIG.  79.  Embryonal  anlagen  of  the  glands  associated  with  the  pharynx 
and  pharyngeal  pockets,  [a-d,  after  MAURER;  e-h,  after  VERDUN.] 

Thymus  anlagen  are  indicated  by  dots,  thyreoid  by  diagonal  lines,  epithelial  cor- 
puscles by  radiating  lines,  and  post-branchial  (supra-pericardial)  bodies  in  black.  The 
pharyngeal  pockets  are  designated  by  Roman  numerals. 

(a)  Cyclostome.  (b)  Teleost.  (c)  Urodele.  (d)  Lizard  (.Lacerta).  (e)  Chick, 
(f)  Cat.  (g)  Man.  (h)  Mole. 


288  HISTORY    OF   THE    HUMAN    BODY 

only  the  last  three  possess  definite  thymus-anlagen,  while  those 
of  the  first  two  are  transitory  rudiments.  In  the  lizard,  with 
four  embryonic  pharyngeal  pockets,  the  first  has  a  rudimentary 
thymus-anlage,  and  the  second  and  third  functional  ones. 
The  fourth  is  without  one.  In  mammals  the  definite  thymus 
is  formed  either  wholly  from  the  anlage  associated  with  the 
third  pocket,  or  else  mainly  from  this  with  the  addition  of  a 
small  contribution  from  the  fourth. 

The  fate  of  the  ventral  anlagen,  that  form  the  epithelial 
corpuscles,  is  known  only  in  part,  but  they  form  at  best  only 
small  nodules,  of  a  more  or  less  glandular  nature,  and  never 
attain  the  bulk  of  a  thymus.  In  amphibians  that  of  the  second 
pocket  becomes  associated  with  the  carotid  artery  and  forms 
the  so-called  "  carotid  gland,"  especially  conspicuous  in  frogs 
and  toads.  Those  of  the  third  and  first  pockets  are  developed 
as  epithelial  corpuscles.  In  the  lizard  the  only  ventral  an- 
lage that  persists  is  that  of  the  third  pocket,  which  forms  a 
carotid  gland.  In  Echidna  epithelial  bodies  appear,  associated 
with  pockets  two  and  four,  that  of  the  second  becoming  a 
carotid  gland  as  in  amphibians.  In  other  mammals  pockets 
three  and  four,  or  pocket  four  alone,  are  provided  with  such 
anlagen.  These  may  remain  associated  with  their  respective 
thymus  anlagen,  or  may  become  detached,  but  possess  little  if 
any  physiological  significance. 

A  third  system  of  organs  associated  with  this  region  ap- 
pears typically  as  a  single  pair  of  evaginations,  which  arise 
from  the  floor  of  the  pharynx,  in  all  cases  immediately  behind 
the  last  gill-slit.  From  their  secondary  relation  to  the  peri- 
cardium in  selachians  they  have  received  the  name  of  siipra- 
pencardial  bodies,  but  they  are  better  termed  the  post-bran- 
chial  bodies,  a  name  which  expresses  a  fundamental  and  uni- 
versal relationship.  In  selachians  both  members  of  the  single 
pair  of  these  organs  develop,  but  in  urodeles  and  in  lizards 
the  left  alone  completes  its  development,  while  the  right  one 
remains  in  a  rudimentary  condition,  and  eventually  disap- 
pears. The  occurrence  of  post-branchial  bodies  in  birds  and 
mammals  is  uncertain,  but  some  identify  with  these  a  pair  of 


THE    DIGESTIVE   AND   RESPIRATORY   SYSTEM     289 

evaginations  that  in  the  latter  class  arise  from  the  same  region 
and  become  eventually  lost  in  the  lobes  of  the  thyreoid  gland, 
the  so-called  parathyreoid  bodies.  Comparing  the  significance 
of  the  post-branchial  bodies,  they  are  looked  upon  by  some  as 
the  rudiments  of  a  pair  of  pharyngeal  pockets  posterior  to  the 
last  definite  ones,  a  point  of  view  which  has  suggested  for  them 
the  name  of  ultimo-branchial  (rather  than  post-branchial) 
bodies ;  but  the  probability  that  the  last  apparent  pocket  in  the 
various  vertebrate  Classes  is  not  always  the  same  one  intro- 
duces a  valid  objection  to  this  hypothesis. 

Still  another  organ  associated  with  the  pharynx  as  primarily 
an  evagination  from  its  walls  is  the  thyreoid  gland.  This, 
in  its  first  appearance,  is  constant  from  the  selachians  to  the 
mammals,  and  arises  as  a  median  outpushing  from  the  floor 
of  the  pharynx  at  a  level  corresponding  to  the  interval  be-? 
tween  the  first  and  second  pockets.  In  its  later  development 
it  becomes,  like  the  thymus,  a  compact  glandular  organ,  with- 
out a  duct,  and  equally  uncertain  and  indefinite  in  its  function, 
but  in  the  larva  of  one  of  the  cyclostomes  (Petromyzori),  we 
catch  a  glimpse  of  its  past  history,  for  here  for  a  time  it  ap- 
pears as  an  open  trough,  lined  with  cilia,  and  in  open  com- 
munication with  the  pharynx.  In  this  condition  it  corresponds 
closely  in  structure  and  position  with  the  hypo-branchial 
groove,  or  end 'o style  of  Amphioxus,  an  organ  which  furthers 
the  passage  of  the  food  down  the  pharynx  by  producing  a 
slimy  secretion  and  by  furnishing  a  ciliated  track  along  which 
the  food  may  be  propelled.  The  thyreoid  gland  was  thus 
primarily  a  digestive  organ  and  deserves  a  place  in  this  chap- 
ter for  reasons  more  fundamental  than  a  mere  topographical 
relation.  In  the  true  vertebrates,  however,  its  function,  with 
its  structure,  becomes  completely  changed,  and  it  no  longer 
assists  in  digestion,  but  presumably  possesses  some  regulating 
effect  upon  the  blood,  especially  that  supplying  the  brain. 
There  is  yet  much  that  is  problematic  in  this,  but  that  the 
association  between  the  thyreoid  and  the  brain  is  an  intimate 
one  is  shown  both  by  the  occurrence  of  cretinism,  a  curious 
developmental  malformation  associated  with  the  thyreoid  and 


290  HISTORY    OF    THE    HUMAN    BODY 

in  which  varying  degrees  of  idiocy  play  a  principal  part,  and 
by  the  frequency  with  which  insanity  has  followed  the  extir- 
pation of  the  thyreoid. 

The  pre-vertebrate  history  of  the  thymus-anlagen  is  less 
certain,  but  attempts  have  been  made  to  homologize  them  with 
the  " tongue  bars"  of  Amphioxus,  gelatinous  rods  used  to 
support  the  gill-clefts.  If  this  be  true  the  change  of  function 
seems  even  greater  than  in  that  of  the  thyreoid,  for  there  is 
more  similarity  between  a  gland-lined  groove  and  a  compact 
glandular  organ,  than  between  the  latter  and  a  skeletal  rod. 
In  its  development  among  the  vertebrates  the  thymus  is  still 
more  problematical  than  is  the  thyreoid;  it  is  usually  volu- 
minous, and  in  many  mammals  extends  along  the  anterior 
mediastinal  space  between  sternum  and  heart  as  far  as  the 
diaphragm.  In  the  human  species  it  is  large  in  childhood,  but 
suffers  regressive  changes ;  during  middle  life  it  is  inconspicu- 
ous, and  in  old  age  is  reduced  to  a  few  rudiments.  •  */r- 
1 '  Beyond  the  pharynx  the  alimentary  canal  becomes  nar- 
rowed and  forms  the  oesophagus,  below  which  it  again  en- 
larges to  form  the  great  expansion  known  as  the  stomach, 
usually  the  most  conspicuous  portion  of  the  entire  tract.  The 
oesophagus  varies  in  length  in  proportion  to  the  length  of 
the  neck  region,  one  extreme  being  represented  by  frogs  and 
toads,  in  which  it  is  reduced  to  a  simple  constriction  like  the 
neck  of  a  bag,  the  other  by  such  cases  as  a  long-necked  bircf. 
In  birds  that  are  graminivorous,  or  those  in  general  which  sub- 
sist upon  hard  or  dry  food,  the  middle  portion  of  the  oesoph- 
agus expands  to  form  a  crop  (ingluvies),  into  which  tne 
food  is  first  collected.  Aside  from  its  use  as  a  receptacle  in 
which  food  may  become  softened  by  soaking,  the  crop  is 
probably  in  part  a  provision  for  the  safety  of  the  birds,  allow- 
ing them  to  greatly  shorten  the  period  of  feeding,  a  time  dur- 
ing which  they  are  preoccupied  and  thus  in  especial  danger 
from  their  enemies. 

>  The  muscular  fibers  of  the  alimentary  canal  usually  change 
from  voluntary  to  involuntary  in  the  upper  part  of  the 
oesophagus,  but  in  some  mammals  striated  fibers,  probably 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     291 

partly  under  the  control  of  the  will,  extend  farther  down,  and 
in  ruminants,  where  the  food  is  voluntarily  disgorged  in  the 
form  of  cuds  for  a  second  chewing,  the  entire  oesophagus 
thus  equipped.  \ 

The  stomach  is  originally  a  simple,  spindle^sliaped  enlarge- 
ment of  the  canal,  extended  lengtjnvise  and  indefinitely  sepa- 
rated from  the  oesophagus,-a4tii6ugh  more  completely  limited 
below  by  a  restriction,  the  pylorus,  which  forms  a  valve  for 
the  purpose  of  temporarily  converting  it  into  a  closed  sac. 
This  typical  form  is  seen  in  fishes  and  tailed  amphibians,  but 
an  attempt  to  increase  its  efficiency  both  in  digestive  surface 
and  in  capacity  causes  the  formation  of  a  curvature  extending 
to  the  left,  so  that  there  is  a  longer  outline  upon  its  left  side 
and  a  shorter  one  upon  its  right,  the  greater  and  lesser  curva- 
tures respectively.  As  the  same  tendency  continues  the 
stomach  turns,  still  to  the  left,  in  such  a  way  that  ultimately 
its  longitudinal  axis  lies  across  the  body,  which  places  the 
upper  or  cardiac  end  on  the  left,  the  lower  or  pyloric  end,  on 
the  right,  the  lesser  curvature  above  and  the  greater  curvature 
below.  Below  the  cardiac  orifice  the  left  end  of  the  stomach 
usually  bulges  out  laterally  to  form  the  fundus,  which  in  some 
cases  becomes  a  more  or  less  distinct  receptacle  for  the  food 
when  first  received ;  and  beyond  approximately  the  middle  the 
stomach  tapers  toward  the  pyloric  end  (right)  and  forms  an 
upward  curve  at  the  culmination  of  which  is  placed  the  pylo- 
rus. This  may  be  considered  the  typical  form  of  mammalian 
stomach  and  is  seen  in  Primates,  Carnrvora,  Insectivora,  and 
Edentata,  these  being  in  other  respects  also  most  primitive 
of  placental  mammals  (Fig.  80,  a). 

Modifications  of  this  primary  form  are  due,  first^to  an  at- 
tempt to  localize  and  define  the  different  portions  of  the  stom- 
ach and  specialize  their  functions,  and,  secondly^  to  various 
attempts  to  increase  the  general  surface  and  thus  develop  a 
greater  physiological  efficiency,  usually  in  connection  with  in- 
nutritious  food  or  with  the  necessity  of  taking  in  a  large 
amount  in  a  short  space  of  time. 

Progress  in  the  first  of  these  directions  is  shown  by  such  a 


HISTORY   OF   THE    HUMAN    BODY 

stomach  as  that  of  the  mouse  (Fig.  80,  b),  in  which  the  con- 
diac  and  pyloric  halves  are  separated  by  a  marked  restriction, 
and  this  tendency  reaches  its  extreme  in  the  ruminants  (Fig. 
80,  h),  where  each  of  these  two  primary  sub-divisions  is  again 
divided,  forming  a  stomach  of  four  compartments,  in  two 
pairs.  The  cardiac  portion  is  divided  into  a  voluminous  paunch 
(rumen),  which  receives  the  food  when  first  taken  in,  and  a 
small,  round  honey-comb  stomach  (reticulum),  in  which  the 
food  from  the  paunch  is  made  up  into  cuds.  The  pyloric  por- 


FIG.  80.     Stomachs  of  various  mammals. 

(a)    Man;    (b)    mouse;    (c)    pig;    (d)    seal;    (e)    vampire   bat;    (f)    manatee;    (g) 
sloth;  (h)  sheep. 

tion  is  divided  into  an  omasus  and  an  abomasus,  into  which 
the  food  passes  in  succession  when  swallowed  a  second  time. 
Local  enlargements  of  surface  frequently  appear  in  the  form 
of  a  prolongation  of  the  fundus  into  one  or  more  diverticula 
(Fig.  80,  e,  f  and  g).  There  are  two  of  these  in  the  hippo- 
potamus; three  in  Tarsipes,  a  small,  insect-eating  marsupial; 
and  in  the  vampire  bat,  Desmodus  (Fig.  80,  e),  the  fundus 
is  elongated  to  form  a  ccecum  of  twice  the  length  of  the  body, 
used  as  a  reservoir  for  blood. 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     293 


It  is  of  interest  in  this  connection  to  trace  the  changes  in  the 
mesentery  of  this  region,  more  precisely  termed  the  mesogas- 
trium,  as  they  appear  in  successive  stages  in  the  mammalian 
embryo  which  are  undoubtedly  of  historical  significance  (Fig. 
81).  The  formation  of  the  initial  curvature  to  the  left  natu- 
rally broadens  the  corresponding  part  of  the  mesogastrium,  an 
effect  still  further  increased  by  the  lateral  torsion  of  the  entire 
stomach.  At  this  point  the  widened  mesentery  comes  under 


ir 


IV 


FIG.  81.  Development  of  the  peritoneal  folds  and  of  the  windings  of 
the  alimentary  canal  in  the  human  embryo.  [From  HERTWIG,  Figure  I 
after  His;  Figure  II  after  TOLDT.] 

Figure  I  shows  the.  spindle-shaped  stomach,  the  lung  anlagen,  and  the  beginning 
of  the  liver  in  the  form  of  a  median  diverticulum;  in  Figure  II  the  peritoneum  is 
shown,  with  pancreas  and  spleen;  figures  III  and  IV  show  the  development  of  the 
omentum  and  the  lesser  peritoneal  cavity. 

the  influence  of  the  spleen,  which  develops  within  it  and  by  its 
weight  produces  a  fullness  which  sags  down  behind  (dorsal 
to)  the  lesser  curvature,  while  attached  to  the  greater;  and 
the  continuation  of  this  tendency  causes  the  free  lower  fold 
of  the  bag-like  extension  to  hang  down  behind  the  contour 
of  the  stomach. 

This  fold  is  the  greater  omentum  (omentum  mafus),  which, 
as  all  mesenteries  are  essentially  double,  consists  of  four  layers 


294  HISTORY    OF    THE    HUMAN    BODY 

of  serous  membranes,  applied  two  and  two,  each  pair  holding 
between  them  the  blood  and  absorbent  vessels  naturally  be- 
longing to  a  mesentery.  The  cavity  of  the  bag  is  the  lesser 
peritoneal  cavity  of  human  anatomy,  and  its  mouth,  opening 
into  it  behind  the  stomach,  is  the  foramen  cpiploicnm  [foramen 
of  Winslow].  In  most  mammals'The  bag  is Twfclely  open,  but 
in  man  the  foramen  is  much  reduced  in  size  and  the  layers 
forming  the  pendulous  fold  are  fused  together,  and  form  a 
four-layered  apron  that  hangs  below  the  stomach  and  covers 
the  intestinal  folds. 

The  remainder  of  the  canal  below  the  pylorus  forms  the  in- 
testine, and  although  this  has  been  divided  for  convenience 
into  several  more  or  less  definite  regions,  they  are  for  the  most 
part  artificial  in  character.  The  most  definite  of  these  are 
the  cloaca  of  Amphibia  and  Sauropsida,  and  the  large  in- 
testine of  mammals  [intestinum  crassuni],  both  enlargements 
of  the  posterior  portion  of  the  intestinal  tract,  but  probably  not 
equivalent  to  one  another;  in  distinction  from  this  the  re- 
mainder is  termed  the  small  intestine  [intestinum  tenue"].  In 
this  latter  the  most  definite  subdivision  is  defined  by  the  en- 
trance of  the  bilary  and  pancreatic  ducts ;  and  the  space  be- 
tween the  pylorus  and  this  point  is  designated  the  duodenum. 
This  portion  often  forms  a  conspicuous  loop,  consisting  of 
ascending  and  descending  limbs,  which  enclose  the  pancreas 
between  them. 

The  liver  and  the  pancreas,  the  two  digestive  glands  asso- 
ciated with  this  region,  are  derived  from  the  mucosa  of  the 
intestines,  from  which  they  arise  as  ev aginations,  the  former 
ventral,  the  latter  dorsal.  As  they  increase  in  size  they  pro- 
trude beyond  the  intestinal  walls  and  force  their  way  between 
the  two  layers  of  their  respective  mesenteries,  as  a  result  of 
which  relation  they  become  invested  with  a  serous  membrane 
continuous  with  that  covering  the  intestines  (peritoneum),  and 
remain  attached  by  ligaments  both  to  the  latter  and  to  the  body 
wall.  These  relations  are  clearly  shown  in  the  accompanying 
diagrams  (Fig.  82),  which  show  the  origin  of  these  organs 
from  the  intestine,  their  serous  investment  and  their  dorsal  and 


THE    DIGESTIVE    AND    RESPIRATORY    SYSTEM     295 


ventral  mesenteries.  In  spite  of  the  great  increase  of  size 
these  typical  relations  remain  in  the  case  of  the  liver,  and  its 
two  suspensory  mesenteries  become  the  ligamcntum  hepato- 
gastriciim  [y~\  (sometimes  termed  the  lesser  omcntnm),  and 
the  ligamentum  suspensoriitm  hepatis  \_x}.  While  primarily 
the  entire  length  of  the  alimentary  canal  that  passes  through 
the  ccelomic  region  becomes  attached  by  both  a  dorsal  and  a 


A 


FIG.  82.  Diagrams  showing  the  relation  of  the  liver  and  pancreas  to 
the  peritoneum.  [After  HERTWIG.] 

(a)  Lateral  view  with  ventral  surface  towards  the  left.  The  organs  are  seen 
lying  within  the  peritoneum,  which  is  represented  in  a  vertical  plane  stretched  across 
from  mid-dorsal  to  mid-ventral  lines,  (b)  A  cross-section.  The  place  through  which 
it  is  taken  is  indicated  approximately  in  (a)  by  the  arrows. 

Organs:  s,  stomach;  n,  spleen;  /,  liver;  p,  pancreas;  i,  intestine.  Ligaments:  x, 
ligamentum  suspensorium  hepatis;  y,  ligamentum  hepatogastrium  (  =  lesser  omentum)  ; 
a  and  b,  parts  of  the  mesogastrium  which  form  the  pancreatic  ligaments  similar  to 
those  of  the  liver. 

ventral  mesentery,  the  ventral  one  becomes  lost  below  the 
region  of  the  liver,  thus  leaving  a  sharp  ventral  edge  to  the 
two  hepatic  ligaments. 

The  gall-bladder  is  formed  as  an  enlargement  of  the  hepatic 
duct  and  is  by  no  means  of  universal  occurrence;  it  develops 
rather  in  response  to  certain  conditions,  much  as  in  the  case 
of  the  crop,  and  its  slight  physiological  importance  is  shown 
by  its  occurrence  in  one  of  two  allied  animals  and  its  ab- 


296  HISTORY   OF    THE    HUMAN    BODY 

sence  in  the  other.  A  good  example  of  this  is  that  of  the 
pigeon  and  the  common  fowl,  in  the  latter  of  which  a  well- 
developed  gall-bladder  occurs  while  absent  in  the  former.  The 
pancreatic  duct  is  normally  without  such  a  resevoir,  but  a  pan- 
creatic bladder  has  occasionally  been  observed  as  an  abnor- 
mality in  the  common  cat,  existing  side  by  side  with  a  normal 
gall-bladder,  the  two  exhibiting  about  the  same  size  and  pro- 
po^tions. 

,/The  terminal  portion  of  the  alimentary  canal  in  Amphibia 
/  and  Sauropsida,  and  in  some  fishes  (e.  g.,  selachians),  enlarges 
into  a  cloacal  chamber  which  bears  within  its  walls  the  out- 
lets of  the  urinary  and  reproductive  organs,  and  receives  their 
products  as  well  as  that  of  the  intestines.  In  the  Sauropsida 
and  in  monotremes  the  terminal  portion  of  this  serves  as  the 
functional  cloaca  and  receives  also  the  urinary  and  reproduc- 
tive products,  but  in  all  mammals  except  these  last  the  uro- 
genital  outlets  are  emancipated  from  the  alimentary  canal, 
which  thus  terminates  in  a  rectum  instead  of  a  cloaca,  and  its 
external  opening  is  a  true  anus  and  not  a  cloacal  orifice. 

At  the  junction  of  the  small  intestine  with  the  large,  there 
is  a  strong  tendency  to  form  one  or  more  cceca,  or  blind  sacs, 
which  often  become  digestive  organs  of  great  physiological 
efficiency.  The  characteristic  form  in  reptiles  is  that  of  a  single 
rather  short  and  wide  ccecum,  symmetrically  placed.  In  birds 
there  are  usually  two  symmetrical  ones,  which  attain  great 
length  in  scratching  birds  (e.  g.f  the  common  fowl),  and  in 
ducks  and  geese,  but  are  quite  rudimentary  in  certain  others 
(woodpeckers,  parrots,  etc.).  Ostriches  possess  a  single 
coecum  of  great  length  (7  to  8  meters)  and  furnished  with 
an  internal  spiral  partition,  which  greatly  increases  its  ef- 
•  fective  surface. 

In  mammals  a  single  coecum  is  developed,*  which  varies 
greatly  in  size  and  functional  importance.     Rudimentary  in 

*  There  are  two  very  short  coeca  in  the  arboreal  ant-eater,  and  in 
the  manatee  a  single  coecum  bears  two  supplementary  diverticula.  In 
Hyrax,  in  addition  to  a  moderately  large  coecum,  there  are  two  smaller 
diverticula  situated  farther  down  on  the  colon. 


THE    DIGESTIVE    AND    RESPIRATORY    SYSTEM     297 

edentates,  most  insectivores,  and  bats,  it  frequently  attains  an 
enormous  size  in  herbivorous  or  graminivorous  forms.  In 
certain  rodents  (e.g.,  muskrat,  woodchuck),  its  total  capacity 
equals  or  exceeds  that  of  the  remainder  of  the  alimentary 
canal,  and  in  the  marsupial  Phascoloarctus  it  is  three  times 
the  length  of  the  body.  In  the  rabbit  it  is  provided  with  an 
internal  spiral  valve ;  in  certain  other  rodents  and  in  the  higher 
apes  and  man,  the  free  end  becomes  rudimentary,  restricts  its 
lumen,  and  forms  a  worm-like  process,  the  processus  (appen- 
dix) vermif ormis ,  which,  like  all  rudimentary  organs,  is  sub- 
ject to  a  large  amount  of  individual  variation. 

Thus  in  the  human  subject  the  appendix  varies  in  length 
between  the  limits  of  2-23  cm.,  the  average  for  an  adult  being 
8-9  cm.  It  is  longest  proportionally  during  fetal  life,  its 
length  relative  to  that  of  the  large  intestine  being  i  :io,  while 
in  adult  life  it  is  1 120.  It  is  longest  absolutely  between  the 
ages  of  ten  and  twenty,  after  which  it  shows  a  slight  reduc- 
tion.* Its.  status  as  a  rudiment  of  slight  functional  value  is 
shown  by  the  tendency  towards  the  obliteration  of  its  lumen, 
a  tendency  which  increases  steadily  with  age.**  Furthermore, 
these  two  characters,  reduction  in  length  and  obliteration  of 
the  lumen,  go  hand  in  hand,  short  appendices  being  usually 
solid,  while  large  ones  are  apt  to  possess  a  lumen. 

The  position  and  arrangement  of  the  colon  varies  consider- 
ably among  various  mammals.  In  man  it  begins  low  down 

*Zuckerkandl  tabulated  the  length  of  the  appendix  in  161  bodies, 
with  the  following  result : 

mm  mm 

17 — 20 2  cases.  90 — 100 15   cases. 

30 — 40 8  100 — no 4 

40—50 6  no— 120 5 

5O — 60 28  120 — I3O 2 

60—70 26      "  130—140 I 

70—80 29      "  140—150 I 

80—90 23      "  150—160 I      " 

**  Wilhelm  Miiller,  from  data  obtained  from  1,005  bodies  dissected  at 
Jena  between  1895  and  1897,  found  the  amount  of  obliteration,  partial 
and  total,  to  be  as  follows:  (See  table  on  p.  298.) 


298 


HISTORY   OF   THE   HUMAN    BODY 


on  the  right  side,  from  which  there  proceed  in  order  an 
ascending,  transverse,  and  descending  portion,  connected  with 
the  rectum  by  a  sigmoid  flexure,  through  which  the  tube  attains 
the  median  line;  a  similar  disposal  is  seen  in  many  other  an- 
thropoids, in  lemurs  and  rodents,  the  majority  of  carnivores, 
and  a  few  others.  A  more  complex  condition  than  this  is  pro- 
duced by  the  formation  of  long,  narrow  loops  along  the 
course  of  either  the  ascending  or  transverse  colons,  or  both, 
and  these  loops  may  remain  simple  or  roll  into  spirals.  Such 
colon  labyrinths  are  seen  in  ruminants,  in  certain  rodents  as 
the  lemmings  and  jumping  mice,  and  in  a  few  lemurs  (Figs. 
83  and  84). 

From  this  brief  review  of  the  alimentary  canal  and  its  modi- 
fications the  impression  is  gained  that  in  this  array  of  enlarge- 
ments, elongations,  diverticula,  spiral  valves,  and  other  de- 
vices, we  have  to  do,  not  with  a  consecutive  anatomical  history, 
but  with  numerous  special  cases  of  physiological  adaptations, 
developed  in  response  to  need;  and  that  a  similarity  in  one  of 

Continued  from   p.   297. 


MALES 

FEMALES 

Age  in 
Years 

No.  of  In- 
dividuals 

No.    of 
Cases  of 
Oblitera- 

Per- 
cent- 

Age in 
Years 

No.  of  In- 
dividuals 

No.  of 
Cases  of 
Oblitera- 

Per- 
cent- 

tion 

age 

tion 

age 

o 

48 

0 

o 

o 

46 

o 

o 

i 

78 

0 

o 

i 

58 

o 

o 

2  —  10 

5i 

i 

2.0 

2  —  IO 

4i 

o 

0 

II  —  20 

39 

2 

5-1 

II  —  2O 

19 

I 

5-4 

21—30 

47 

3 

6.4 

21—30 

23 

2 

8-7 

31—40 

55 

7 

12.7 

31—40 

38 

9 

23.8 

41—50 

84 

22 

26.2 

41—50 

46 

16 

34.8 

5r_6o 

73 

15 

20.5 

51—60 

60 

18 

300 

6  1  —  70 

58 

17 

2-9-3 

61  —  70 

48 

24 

50.0 

71—80 

3i 

12 

38.7 

71-80 

30 

8 

26.6 

81—90 

15 

8 

53-3 

81—90 

i? 

9 

52.9 

579 

426 

Comparison  of  the  percentage  columns  shows  that  in  women  there 
exists  a  greater  tendency  towards  obliteration  than  in  men.  The  few  dis- 
crepancies in  the  table,  for  example,  the  smaller  average  given  for  men 
between  50  and  60,  and  in  women  between  70  and  80,  are  doubtless  due  to 
the  small  number  of  individuals  examined. 


THE    DIGESTIVE   AND    RESPIRATORY   SYSTEM     299 

these  particulars  implies,  not  genetic  relationship  necessarily, 
but  a  similar  demand  responded  to  in  a  similar  way.  The 
main  object  to  be  achieved  in  all  cases  is  to  regulate  the  amount 
of  digestive  surface  to  the  demands  offered  by  the  various 
kinds  of  food,  and  as  there  is  but  a  limited  number  of  me- 
chanical or  architectural  devices  possible,  the  same  ones  are 
employed  in  unrelated  groups  of  animals,  having  arisen  in- 
dependently in  response  to  a  similar  physiological  need.  This 
phenomenon  of  parallel  development  (or  "  analogical  resem- 


\ 


FIG.  83.    Colon  labyrinth  of  Ceruus  canadensis.     [After  WEBER.] 

blance,"  as  Darwin  calls  it),  may  appear  in  any  system  or  part 
and  has  been  a  frequent  source  of  error  in  the  estimation  of 
the  inter-relationship  of  animals. 

The  relation  of  the  total  length  of  the  intestine  to  the  kind 
of  food  has  been  frequently  emphasized,  the  idea  prevailing 
that  it  is  short  in  carnivores  and  long  in  herbivorous  forms,  in 
accordance  with  the  difference  in  nutrient  qualities  and  the 
ease  of  digestion  in  the  two  sorts  of  food,  but  this  statement 
is  to  be  accepted  only  in  a  general  way,  as  it  is  subject  to 


300  HISTORY   OF    THE    HUMAN    BODY 

modifications  through  the  compensation  furnished  by  other 
factors,  such  as  the  special  devices  just  considered.  Thus  in 
the  ox  the  length  of  the  entire  intestine,  small  and  large,  in 
proportion  to  the  length  of  the  body  taken  as  unity  is  20:1, 


FIG.    84.      Colon    labyrinth    of    Propithecus    diadema.      [From    WEBER, 
after  VAN  LOGHEM.] 

while  in  the  horse,  which  eats  similar  foods,  it  is  but  12:1,  but 
in  this  latter  animal  an  enormously  developed  coecum  furnishes 
a  compensation  for  the  reduction  in  length  of  the  main  canal. 
Perhaps  the  greatest  extremes  of  variation  within  the  same 
Order  are  shown  within  the  limits  of  the  Cetacea,  where  the 
proportionate  length  varies  between  4:1  and  32:1.  This  last, 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     301 

probably  the  largest  among  mammals,  is  reported  to  be  that  of 
Pontoporia,  a  South  American  dolphin,  while  the  shortest 
mammalian  intestine  is  that  found  in  certain  insectivorous 
bats,  the  proportion  of  which  to  the  body  length  is  2:1.  A 
change  in  the  length  or  volume  of  an  organ  is,  however,  so 
easily  effected  even  during  an  animal's  lifetime,  that  it  is  prob- 
able that  members  of  the  same  species  may  show  considerable 
difference  in  length  of  intestine,  especially  if  a  comparison  be 
made  between  specimens  from  quite  different  localities  where 
the  diet  is  different.  Thus  in  man,  the  intestinal  canal  of  the 
Japanese,  whose  food  is  largely  vegetable,  exceeds  in  average 
length  by  one-fifth  that  of  Europeans;  and  in  whites  of  me- 
dium stature  the  length  of  the  intestine  proper,  from  pylorus 
to  anus,  averages  960  cm.,  while  the  average  length  of  the 
same  in  nine  negroes  was  but  866.7  cm. 

This  whole  subject,  therefore,  gives  but  little  indication  of 
phylogeny  and  is  valuable  in  the  present  inquiry  mainly  as  an 
example  of  the  complete  correlation  between  environment  and 
structure.  In  the  examples  given  mammals  have  been  pur- 
posely emphasized  and  instances  of  adaptations  in  the  other 
groups  of  vertebrates  have  been  omitted  as  far  as  possible, 
since  their  inclusion  would  convey  the  subject  far  beyond  the 
proper  limits  of  this  work. 

The  function  of  respiration  is  the  simplest  of  the  major 
functions,  since  it  consists  primarily  of  an  interchange  of  gases 
through  osmosis,  and  involves  in  itself  nothing  save  a  moist 
membrane,  with  air  or  aerated  water,  on  one  side  and  blood 
on  the  other.  The  blood  must  be  constantly  renewed  through 
some  form  of  circulation,  and  there  is  usually  some  auxiliary 
mechanism  to  create  a  current  in  the  respiratory  medium  also. 
It  is  also  imperative  that  the  osmotic  membrane  be  kept  moist, 
a  matter  of  no  difficulty  in  an  aquatic  animal,  but  one  involving 
some  little  additional  apparatus,  usually  an  interior  chamber 
with  a  regulated  outlet,  in  terrestrial  forms.  Thus  in  aquatic 
invertebrates  the  respiratory  membrane  is  usually  external, 
often  a  modified  portion  of  the  integument.  In  many  minute 
forms  in  which  the  integument  is  thin,  respiration  takes  place 


302  HISTORY   OF   THE    HUMAN    BODY 

through  the  general  surface  without  the  formation  of  localized 
organs  for  the  purpose,  and  in  larger  forms  effective  organs 
of  respiration  are  produced  by  the  formation  of  external  folds 
or  other  outpushings  of  the  integument.  These  are  formed 
in  the  embryo  when  the  skin  is  still  soft  and  thin  and  remain 
in  the  adult  state  unaffected  by  the  process  of  chitinization 
which  involves  the  surrounding  integument.  Such  organs  are 
called  gills,  a  general  term  for  all  aquatic  respiratory  organs. 
These  present  numerous  mechanical  devices  for  increasing  the 
surface ;  they  may  be  in  the  form  of  single  plates,  sets  of  plates 
placed  parallel  to  one  another,  dendritic  structures  formed  by 
the  repeated  branching  of  simple  diverticula,  sets  of  parallel 
tubes  for  the  blood  with  interspaces  for  the  water,  and  so  on, 
and  are  in  most  cases  provided  with  accessory  structures, 
some  for  protection  and  others  for  producing  a  current  of 
water. 

In  a  terrestrial  animal,  on  the  other  hand,  the  respiratory 
system  must  be  internal  in  order  to  secure  the  proper  con- 
ditions of  moisture,  and  as  all  terrestrial  animals  are  the 
descendants  of  aquatic  ones  that  succeeded  in  adapting  them- 
selves to  the  difficult  environment  of  land,  with  its  many  dis- 
advantages, it  forms  an  interesting  study  in  adaptation  to 
compare  the  respiratory  system  in  each  terrestrial  group  with 
that  of  the  animals  which  most  nearly  represent  their  aquatic 
ancestors.  In  some  cases  the  old  respiratory  organs  are  re- 
tained by  sinking  them  into  deep  recesses  kept  moist  by  glands, 
in  others  they  are  discarded  in  favor  of  new  ones,  formed,  per- 
haps, by  the  transformation  of  some  ready-to-hand  cavity, 
which  is  lined  with  blood  vessels  and  made  to  communicate 
with  the  exterior  through  some  regulated  outlet.  Still  another 
principle  is  seen  in  the  tracheal  tubes  of  insects,  which  are 
branching  tubes  lined  with  chitin  and  leading  from  a  series 
of  external  openings  into  the  interior,  ramifying  all  the  inter- 
nal organs.  These,  as  shown  by  their  development,  are  in 
origin  integumental  folds,  like  the  plate-like  gills  of  their  an- 
cestors, which,  as  they  develop,  turn  in  instead  of  out,  thus 
satisfying  the  conditions  of  aerial  respiration. 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     303 

Essentially  different  in  principle  from  all  of  the  respiratory 
methods  thus  far  mentioned  is  that  which  utilizes  for  the 
purpose  some  portion  of  the  wall  of  the  alimentary  canal,  a 
method  employed  sporadically  among  invertebrates,  especially 
the  echinoderms,  and  forming  the  essential  system  in  verte- 
brates and  allied  forms.  In  this,  which,  by  using  the  term  in 
its  most  comprehensive  sense,  may  be  called  intestinal  respi- 
ration, the  function  is  usually  located  near  one  end  of  the  ali- 
mentary canal,  for  the  purpose  of  obtaining  the  respiratory 
medium,  and  the  wall  at  this  place  is  richly  supplied  with 
capillaries,  through  which  the  interchange  of  gases  takes 
place.  The  respiratory  medium,  which  may  be  either  air  or 
water,  is  kept  in  motion  by  a  system  of  involuntary  or  semi- 
voluntary  muscles,  and  the  motion  thus  generated  is  usually 
rhythmic  in  character. 

In  the  vertebrates,  as  well  as  in  those  invertebrates  that 
probably  represent  their  ancestors,  the  respiratory  function  is 
located  in  the  pharynx  and  the  respiratory  current  is  primarily 
taken  in  at  the  mouth  and  driven  out  through  a  series  of  lateral 
openings,  the  gill-slits.* 

These  latter,  as  seen  in  the  worm-like  Balanoglossus,  and 
in  Amphioxus,  as  well  as  in  the  embryos  of  true  vertebrates, 
are  seen  to  be  metameric  in  character,  a  pair  for  each  somite, 
and  to  be  arranged  in  a  single  row  along  each  side ;  but  in  the 
more  specialized  group  of  Tunicata,  these  rows  of  slits  which 
appear  in  the  larva  become  secondarily  modified  by  the  forma- 
tion of  numerous  cross-bars,  so  that  ultimately  the  entire 
pharynx  comes  to  resemble  a  grating  or  a  loosely  woven 
basket. 

The  number  of  these  slits  is  very  large  in  both  Balanoglos- 
sus and  Amphioxus,  but  has  suffered  a  considerable  reduction 

*  Aside  from  the  respiration  at  the  anterior  end  of  the  canal  there  are 
a  few  isolated  instances  of  respiratory  action  in  other  parts  of  its  extent. 
Thus  in  the  teleost  Cobitis,  an  Eastern  Hemisphere  carp,  some  respira- 
tory function  is  possessed  by  the  intestine ;  and  in  turtles,  two  lateral 
bladders,  opening  from  the  cloaca  in  association  with  the  median,  or 
allantoic  bladder,  are  used  for  aquatic  breathing. 


chics.  When,  in  the  history  of  the  race,  vertebrates  came  out  of 
the  water  and  adapted  themselves  to  a  terrestrial  element,  they 


304  HISTORY    OF    THE    HUMAN    BODY 

in  true  vertebrates,  so  that  even  in  the  lowest  of  the  fishes  no 
more  than  nine  pairs  are  ever  indicated,  and  this  number 
suffers  a  constant  reduction  in  higher  forms,  the  loss  being 
progressively  from  behind  forwards.  In  the  lower  vertebrates 
also  the  effectiveness  of  the  system  is  increased  by  the  forma- 
tion, in  the  endoderm  lining  the  pharynx,  of  soft  structures, 
richly  supplied  with  blood  vessels,  which  border  the  gill-slits 
and  form  the  true  respiratory  organs,  the  definite  gills  or  bran- 

\ 

substituted  for  this  branchial  system  a  pulmonary  one,  em- 
ploying as  lungs  a  pair  of  sacs  which  open  into  the  floor  of  the 
pharynx  a  little  behind  the  last  gill-slits,  and  which  were  un- 
doubtedly in  existence  at  the  time  of  the  change,  employed  as 
air  bladders.  In  the  gradual  perfection  of  this  second  respira- 
tory system  many  of  the  parts  of  the  old  one  obtained  employ- 
ment, and  were  one  after  another  selected  and  modified  to  add 
to  its  efficiency. 

This  history  of  the  sudden  replacement  of  one  system  by 
another,  and  of  the  gradual  perfection  of  the  second  by  making 
over  to  its  own  use  the  material  of  the  first,  forms  one  of  the 
most  interesting  although  most  complex  bits  of  anatomical 
history,  and  one  of  which  the  record  has  been  especially  well 
preserved.  As  it  involves,  however,  the  entire  region  and  in- 
cludes skeletal  parts,  muscles,  nerves  and  other  elements  aside 
from  those  which  may  be  strictly  termed  respiratory  organs, 
much  of  the  history  will  be  found  in  the  chapters  devoted  to 
those  other  parts.  Here  an  attempt  will  be  made  to  outline 
the  history  of  the  parts  as  a  whole,  with  special  reference  to 
the  function  of  respiration. 

The  fish  type  of  respiratory  apparatus  is  presented  in  its 
most  primitive  form  in  the  sharks  and  dog-fish,  since  numerous 
modifications  which  have  been  acquired  in  the  more  specialized 
fish  are  absent.  It  is  a  type  that  looks  both  ways,  and,  while 
in  many  respects  similar  to  that  of  Amphioxus,  from  it  may  be 
clearly  derived  the  branchial  respiratory  system  of  higher 
forms.  Like  all  special  respiratory  organs  of  vertebrates,  it 


THE    DIGESTIVE    AND    RESPIRATORY    SYSTEM     305 


is  essentially  pharyngeal  and  consists  primarily  of  a  series  of 
lateral  pockets  in  the  walls  of  the  pharynx,  which  break 
through  to  the  exterior  and  form  slits.  These  openings  are 
metameric  in  arrangement  and  are  paired,  each  pair  corre- 
sponding to  a  single  metamere  as  expressed  in  the  associated 
organs,  but  show  considerable  reduction  in  number  from  those 
found  in  Amphioxus,  the  functional  slits  in  most  cases  being 
limited  to  five  pairs.  In  two  especially  primitive  genera,  how- 


FIG.  85.    Respiratory  organs. 

(a)    Cyclostome.      (b)    Selachian,      (c)    Teleost.      (d)    Selachian  embryo,      (e)    Am' 
phiuma    larva.       (f)     Cryptobranchus    larva     [from    a    Japanese    print]. 
n,   nostril;   s,   spiracle;   g,   gill-slits. 

ever,  Hexanchus  and  Heptanchus,  there  are  respectively  six 
and  seven,  facts  which  suggest  that  the  number  at  present  rep- 
resents a  reduction  from  a  previously  more  extensive  series, 
the  reduction  being  from  behind  forwards.  Upon  the  pharyn- 
geal side  of  these  slits  there  develops  a  series  of  soft  organs  in 
the  form  of  fringes  or  tubes,  which  consist  of  localized  elabora- 
tions of  the  pharyngeal  wall,  the  gills  or  branchice.  These 
are  profusely  vascular  and  are  supplied  with  a  rich  capillary 
network  developed  between  two  sets  of  branchial  arteries,  the 


306  HISTORY   OF   THE    HUMAN    BODY 

afferent  branchials,  which  bring  the  blood  directly  from  the 
heart  to  the  gills,  and  the  efferent  branchials,  which  collect  the 
blood  from  the  gill  capillaries  and  unite  to  form  the  main 
aorta.  Since  the  blood  is  aerated  in  the  capillary  network  of 
the  gills  it  follows  that  the  blood  coming  from  the  heart 
through  the  afferent  vessels  is  impure,  while  that  in  the  effer- 
ent vessels  is  pure ;  and  since  these  latter  unite  to  form  the  main 
aorta,  this  vessel,  the  branches  of  which  supply  the  entire  body, 
contains  only  aerated  blood,  while  the  heart  is  employed  merely 
to  receive  the  venous  blood  which  returns  from  all  parts,  and 
to  send  it  to  the  gills.  This  is  the  primitive  type  of  vertebrate 
circulation,  and  obtains  not  only  in  all  fishes,  but  reappears  as 
the  early  form  in  all  vertebrate  embryos,  thus  proving  its 
fundamental  character  as  an  historic  stage. 

The  essential  respiratory  cavity  is  thus  the  entire  pharynx, 
through  which  a  current  of  water  is  kept  in  constant  flow  by 
being  taken  in  at  the  mouth  and  exhaled  through  the  gill- 
slits;  and  while  in  earlier  forms,  as  suggested  by  Ampkioxus, 
the  capillaries  lie  in  the  unmodified  pharyngeal  wall  in  the 
vicinity  of  the  gill-slits,  the  selachians  show  a  considerable  ad- 
vance by  the  formation  of  definitely  localized  organs,  with  a 
Jarge  increase  of  surface,  and  thus  physiologically  more  effi- 
cient. 

In  other  fishes  this  gill-system,  which  is  essentially  similar  to 
the  foregoing,  exhibits  several  secondary  modifications,  the 
most  apparent  of  which  is  the  formation  of  a  large  gill-flap 
(operculum) ,  which  arises  in  front  of  the  most  anterior  slit 
and  extends  backwards,  and  as  the  slits  become  closely  ap- 
proximated and  are  reduced  in  number  to  four,  the  operculum 
becomes  capable  of  closing  entirely  over  them,  meeting  a  ridge 
of  integument  behind  the  last  slit  (Fig.  85,  a).  The  current 
of  water  is  directed  by  rhythmic  respiratory  movements,  which 
consist  of  opening  and  closing  the  mouth  and  operculum,  the 
motions  of  the  two  alternating  with  each  other. 

With  the  fishes  true  internal  (endodermic)  gills  pass  away, 
but  in  the  permanently  aquatic  salamanders  and  in  all  larval 
amphibians  one  or  more  slits  break  through,  usually  two  to 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     307 

three,  in  connection  with  which  certain  integumental  struc- 
tures arise  which  are  gills  physiologically,  but  are  unrelated  to 
the  former.  The  most  widely  distributed  form  of  these  is 
that  of  the  external  branchic?,  three  in  number  upon  each  side, 
and  attached  to  the  cartilaginous  gill-arches.  In  structure 
they  are  usually  plumose  or  dendritic  (Fig.  85,  e  and  f), 
although  in  a  few  cases  they  are  thin  and  leaf-like.  The  slits 
appear  between  these,  with  occasionally  an  additional  one  in 
front  of  the  first,  and  the  animals  obtain  fresh  water  for  res- 
piration in  part  by  forcing  a  current  through  these  slits  in  the 
manner  of  fishes,  and  in  part  by  waving  the  branchiae  up  and 
down  by  means  of  special  muscles  with  which  these  organs 
are  furnished.  As  stated  above,  external  branchiae  are  char-  | 
acteristic  of  the  larvae  of  all  amphibians,  and  are  found  per-  j 
manently  in  a  few  aquatic  salamanders,  which  are  either  more 
primitive  than  the  rest,  or  are  paedogenetic,  that  is,  they  retain 
the  larval  form  while  becoming  sexually  mature.  These  sala- 
manders are  called  perennibranchiate  in  distinction  from  those 
in  which  the  branchiae  become  lost,  the  caducibranchiate  sala- 
manders. A  second  form  of  gills  which  are  external  in  origin 
but  become  internal  in  position,  occurs  in  frog  larvae,  where 
they  replace  the  former,  which  appear  at  first.  As  these  are 
plate-like  and  are  attached  to  the  gill-arches,  they  have  often 
been  considered  exactly  homologous  with  the  gills  of  fishes, 
but  their  ectodermic  origin  renders  such  a  conclusion  impos- 
sible. 

Aside  from  the  two  sets  of  branchiae  most  amphibians  pos- 
sess definite  lungs,  which  arise  in  the  larvae  and  exist  for  a 
time  side  by  side  with  the  external  branchiae,  usually  replacing 
them  in  later  life.  These  are  often  in  the  form  of  simple 
sacs,  without  any  formation  of  internal  partitions,  and  even 
when  in  their  highest  development,  as  in  frogs,  are  far  from 
complex.  It  thus  seems  probable  that,  although  they  are  true 
lungs  anatomically,  they  play  a  subordinate  role  in  respiration, 
and  are  perhaps  primarily  used  either  for  regulating  the  spe- 
cific gravity  of  the  animal  in  the  water,  or  in  the  production 
of  the  voice,  since  the  larynx  is  often  very  large  and  curiously 


308  HISTORY   OF   THE    HUMAN    BODY 

specialized,  and  is  of  considerable  importance  in  the  produc- 
tion of  sexual  calls.  The  slight  respiratory  importance  of  the 
lungs  in  amphibians  is  further  emphasized  by  the  fact  that  in 
a  large  number  of  species  of  salamanders,  both  lungs,  trachea 
and  larynx  are  entirely  wanting,  although  in  a  few  cases  rudi- 
ments of  these  parts  attest  the  former  presence  of  these  organs. 

The  question  will  naturally  occur  at  this  point:  what  are 
the  means  of  respiration  in  adult  amphibians  if  they  have  lost 
their  branchiae  and  yet  possess  either  no  lungs  at  all  or  those 
of  slight  functional  importance?  The  answer  to  this  lies  in 
the  fact  that  amphibians  have  developed  two  other  systems, 
neither  branchial  nor  pulmonary,  the  assumption  of  which 
shows  how  great  may  be  the  systematic  response  to  a  physi- 
ological need,  and  suggests  also  the  trying  period  of  transition 
when  vertebrates  first  essayed  the  terrestrial  environment,  and 
when  attempts  were  made  in  all  possible  directions  to  adapt 
themselves  to  the  new  respiratory  medium.  These  two  sys- 
tems are  the  integumental  and  the  pharyngo-cesophageal,  and 
as  both  of  these  demand  for  their  highest  efficiency  a  moist 
environment  with  an  occasional  submersion  in  water,  they  are 
successful  in  amphibians  with  their  semi-aquatic  mode  of  life, 
but  in  higher  forms  have  been  discarded  in  favor  of  the  pul- 
monary system,  which  enables  its  possessor  to  leave  the 
marshes  and  inhabit  the  dry  land. 

The  origin  of  the  assumption  of  a  respiratory  function  by 
the  amphibian  skin  may  be  traced  to  the  abundance  of  integu- 
mental glands,  inherited  from  the  fishes  and  used  to  protect 
the  surface  from  the  action  of  the  water.  The  presence  of 
these  glands  necessitates  the  formation  of  a  superficial  net- 
work of  capillaries  to  supply  them  with  nourishment,  and  the 
integument  becomes  thus  transformed  into  an  organ  that  pos- 
sesses the  qualities  necessary  for  a  respiratory  organ,  that  is, 
a  moist  surface  bathed  by  the  respiratory  medium  and  sup- 
plied with  a  rich  capillary  net-work.  Thus  apparently  by  ac- 
cident, as  in  all  morphological  changes,  an  organ  which  be- 
comes modified  for  a  certain  function  shows  a  capability  of 
assuming  a  second  one,  not  intended  in  the  original  plan,  and 


THE    DIGESTIVE   AND    RESPIRATORY    SYSTEM     309 

the  moist,  glandular  skin  becomes  an   effective   respiratory 
organ. 

The  pharyngo-cesophageal  system  appears  to  be  a  special 
compensation  for  the  loss  of  the  lungs,  and  is  present  in  only 
those  salamanders  in  which  the  pulmonary  system  has  been 
lost  (Fig.  86).  Here  again  the  incentive  towards  the  for- 
mation is  a  moist  membrane  richly  supplied  with  capillaries, 


Pharyngeal,. 
Muscle 


Gesophageal 
Muscle 


Art.Pharyngea 

,  Portion,  of 
PulmonaiyArcK 

/Art.Gastrica 

\anasiomoses 

[heremththe 

\pulraoiiaryArch 

Art.Gastrica 
V.  oesophagea 


FIG.  86.  Pharyngo-cesophageal  lung  of  Desmognathus,  showing  pha- 
ryngeal  and  oesophageal  muscles,  and  the  net-work  of  blood-vessels  in  the 
walls  of  the  pharynx  and  oesophagus. 


in  this  case,  the  mucous  lining  of  the  pharynx  and  oesophagus. 
The  natural  vascularity  of  this  structure  has  been  considerably 
increased,  while  the  capillaries  themselves  have  become  more 
superficial  and  even  invade  the  external  epithelial  layer,  the 
only  case  known.  The  muscles  of  the  lost  pulmonary  system 
have  been  in  part  retained,  and  through  their  aid,  together  with 
that  of  others  which  are  developed  for  that  purpose,  the 


310  HISTORY   OF   THE    HUMAN    BODY 

pharynx  and  oesophagus  are  dilated  and  contracted  in  asso- 
ciation with  the  usual  respiratory  movements  of  nostrils  and 
floor  of  the  mouth,  and  the  anterior  part  of  the  alimentary 
canal  thus  becomes  a  functional  lung  with  the  power  of  in- 
spiration and  .expiration,  forming  doubtless  a  more  efficient 
organ  than  the  simple  air-sacs  which  these  salamanders  allowed 
to  atrophy. 

Above  the  amphibians,  which,  with  their  numerous  methods 
of  respiration,  suggest  the  experimentation  of  our  early  an- 
cestors in  their  attempts  to  occupy  what  must  have  been  at 
first  an  unnatural  environment,  the  pulmonary  system  becomes 
supreme,  and  its  further  development  is  shown  principally  in 
the  increased  efficiency  of  its  two  main  organs,  the  lungs  and 
the  larynx.  The  later  history  of  this  system  is  quite  well 
known,  especially  that  of  its  development  in  terrestrial  verte- 
brates, but  the  origin  of  the  system  is  still  in  part  obscure, 
and  rests  upon  surmises  rather  than  upon  actual  proof. 

The  history  begins  with  the  period  represented  by  fishes, 
during  which  the  pharynx  exhibits  a  tendency  to  throw  off 
median  diverticula,  sometimes  dorsal  and  sometimes  ventral, 
for  the  purpose  of  forming  pneumatic  cysts  or  air-bladders 
to  add  to  the  buoyancy  and  thus  aid  in  swimming.  In  many 
cases  these  become  closed  and  depend  upon  the  adjacent  blood 
vessels  for  the  gases  with  which  they  become  distended,  but 
in  others  the  original  connection  with  the  pharyngeal  cavity 
is  retained  and  the  two  are  kept  in  communication  through  a 
small  duct.  In  this  latter  case  the  cyst  is  filled  with  air,  which 
is  expelled  and  renewed  through  the  mouth  when  the  fish  is 
at  the  surface  of  the  water,  a  proceeding  that  demands  some 
sort  of  regulator  at  the  orifice  of  the  duct,  an  opening  to  which, 
by  an  extension  of  meaning,  the  term  glottis  may  be  applied. 
Such  an  apparatus,  which  consists  of  muscles  and  fibro-carti- 
lage,  is  a  functional  larynx,  of  which  there  must  be  two  distinct 
organs,  a  larynx  dorsalis,  and  a  larynx  ventralis,  in  accordance 
with  the  position  of  the  pneumatic  cyst.  That  cysts  in  these 
two  positions  cannot  be  homologous  is  evident;  indeed,  those 
in  the  same  position  in  fish  not  closely  related  are  not  neces- 


THE    DIGESTIVE   AND   RESPIRATORY    SYSTEM     311 

sarily  the  same,  yet  until  the  subject  has  been  thoroughly  in- 
vestigated, the  latter  may  be  assumed  to  be  the  case. 

In  several  ganoids  either  one  or  the  other  of  these  pneu- 
matic systems  becomes  complex  in  character  and  serves  as  an 
accessory  respiratory  organ.  The  air-bladder  functions  as  a 
lung;  it  becomes  honeycombed  with  connective  tissue  parti- 
tions, and  is  profusely  vascular,  thus  forming  an  organ  of  far 
greater  functional  activity  than  the  definite  lungs  of  many 
amphibians ;  corresponding  to  this  its  larynx,  the  regulator  of 
the  air  supply,  develops  an  extensive  set  of  muscles  and  masses 
of  fibro-cartilage.  That  such  a  structure,  when  dorsal  in 
position,  as  in  Lepisosteus,  cannot  be  the  precursor  of  the 
final  pulmonary  system  of  higher  forms  is  self-evident,  but 
when  such  an  organ  is  ventrally  placed,  thus  corresponding 
exactly  to  the  embryonic  stages  of  the  latter,  as  in  Polypterus, 
such  an  homology,  although  not  definitely  proven,  is  very 
likely.  As  for  the  dorsal  system,  there  is  no  indication  that 
it  is  represented  in  any  way  above  the  fishes. 

If,  however,  the  ventral  air-bladder  of  Polypterus  is  identi- 
cal with  the  paired  lungs  of  higher  forms  (which  begin  as  a 
single  median  diverticulum  that  divides  later  into  two 
branches),  the  larynx  can  be  the  same  only  in  respect  to  its 
opening,  the  glottis,  since  the  accessory  parts,  that  form  the 
functional  organ,  are  derived  from  two  totally  different  sources 
in  the  two  cases.  In  the  fish  larynx  the  hard  parts  are  derived 
from  the  adjacent  connective  tissue,  and  are  composed  of 
fibro-cartilage,  which  represents  as  it  were  the  first  stage  in 
cartilage  formation  and  differs  but  little  from  a  compact  form 
of  simple  connective  tissue.  The  muscles  are  evidently  slips 
differentiated  from  the  muscular  walls  of  the  pharynx.  That 
this  forms  a  very  effective  organ  cannot  be  denied  and,  had 
no. better  material  for  a  laryngeal  mechanism  been  furnished, 
that  of  the  ganoid  with  its  fibro-cartilage  and  slips  of  pharyn- 
geal  muscle  would  have  undoubtedly  developed  to  fill  all  the 
needs  of  a  pulmonary  system,  even  including  the  functions  of  \ 
voice  and  speech.  It  chanced,  however,  that  at  this  period, 
the  fifth  branchial  arches  with  their  accompanying  muscles, 


312  HISTORY   OF   THE    HUMAN    BODY 

emancipated  from  all  respiratory  function,  and  employed  in  a 
desultory  way  as  tooth-bearing  structures  or  as  parts  assisting 
in  deglutition,  were  lying  in  the  immediate  neighborhood,  one 
on  each  side  of  the  glottis  but  a  little  anterior  to  it,  and 
equipped  with  well-differentiated  muscles ;  and  it  may  well  have 
happened  that  little  by  little  these  parts  may  have  usurped  the 
function  of  the  other  apparatus,  being  better  fitted  for  the 
purpose. 

Be  that  as  it  may,  when,  after  a  succession  of  forms  that 
have  become  lost,  the  curtain  rises  upon  the  lowest  of  the 
amphibians,  this  very  pair  of  arches  is  seen  lying,  one  upon 
each  side  of  the  glottis,  and  forming  with  its  muscles  a  primi- 
tive though  very  effective  larynx.  These  cartilages  are 
proven  to  be  the  actual  5th  pair  of  gill-arches  through  the  iden- 
tity of  their  nerve  supply,  and  the  weak  point  in  the  story  is  the 
identity  of  the  two  pulmonary  systems,  that  of  the  ganoid  and 
the  definite  one  found  in  terrestrial  vertebrates,  a  point  not  yet 
proven;  but,  granting  this,  a  theory  which  seems  extremely 
probable,  the  rest  must  follow.  In  all  events  the  history  of 
both  lungs  and  larynx  from  the  amphibians  on  is  a  continuous 
one,  and  the  latter  organ,  equipped  at  the  start  with  the  $th 
branchial  cartilages  and  their  associated  parts,  becomes  more 
complex  by  the  gradual  addition  of  other  arches,  proceeding 
from  behind  forwards,  each  accommodating  itself  in  shape 
and  position  to  the  especial  function  desired  in  each  case. 

The  simplest  amphibian  larynx  is  that  of  the  perenni- 
branchiate  salamander  Necturus,  where  the  two  cartilages 
in  question  are  in  the  form  of  flattened  triangular  pieces,  the 
lateral  cartilages,  placed  one  upon  each  side  of  the  glottis 
(Fig.  87,  a).  A  short  membranous  trachea,  entirely  without 
cartilaginous  support,  leads  to  the  bag-like  lungs.  In  an 
allied  form,  Proteus  (Fig.  87,  b),  a  slight  advance  is  seen  in 
the  fact  that  the  posterior  angles  of  the  lateral  cartilages  are 
more  prolonged  and  appear  as  slender  processes  which  are 
applied  along  the  sides  of  the  entire  trachea  as  far  as  the 
bronchi.  These  in  adult  animals  show  a  tendency  to  separate 
from  the  main  mass.  This  differentiates  the  cartilaginous 


THE   DIGESTIVE   AND    RESPIRATORY    SYSTEM     313 

pieces  into  an  anterior  pair  of  arytanoids,  upon  either  side 
of  the  glottis,  and  a  posterior  pair  of  tracked  pieces.  Within 
the  Class  of  Amphibia  there  are  no  new  pieces  formed  beyond 
these,  but  they  exhibit  a  great  variety  of  forms,  and  become 
especially  complex  in  the  Anura,  where  they  are  employed  in 
the  production  of  various  sorts  of  notes  used  as  sexual  calls 
(Fig.  87,  c-e).  The  muscles  associated  with  these  skeletal 
elements  consist  originally  of  a  pair  of  dilatators,  which  are 
attached  to  the  outer  edges  of  the  cartilages  and  serve  to 
draw  them  apart,  and  a  double  pair  of  adductors,  the  laryngei, 
which  stretch  across  from  one  to  another  and  serve  to  approxi- 
mate them.  These  give  rise  in  many  of  the  more  complicated 

A 


a  b  c  d  e 

FIG.  87.     Laryngeal  cartilages  of  various  Amphibians. 

(a)  Necturus  (mud-puppy),  (b)  Proteus.  (c)  Amphiuma.  (d)  Triton  (Newt), 
(e)  Rana  (frog). 

cases  to  an  entire  system  of  muscles,  mainly  connected  with 
the  arytaenoid  cartilages,  which  form  the  essential  skeletal  or- 
gan of  the  larynx,  and  to  which  the  vocal  cords  in  the  form 
of  mucous  folds  become  attached. 

In  the  Sauropsida  there  are  two  conspicuous  points  of  ad- 
vance ;  the  one  concerns  the  larynx,  the  other  the  trachea.  The 
first  consists  of  the  addition  of  the  4th  pair  of  branchial  car- 
tilages, which  become  reduced  in  size,  unite  in  the  middle  and 
form  a  triangular  flap,  the  epiglottis;  this,  during  passive 
breathing,  stands  erect  above  the  glottis  but  shuts  down  over 
the  latter  during  the  act  of  swallowing,  thus  preventing  the 
entrance  of  solid  food  into  the  trachea.  The  second  advance 
consists  of  the  presence  of  a  series  of  rings  of  approximately 


314  HISTORY    OF    THE    HUMAN    BODY 

equal  size,  which  embrace  the  trachea  and  protect  it  from 
collapse.  These  are  deficient  behind,  where  the  trachea  comes 
in  contact  with  the  oesophagus,  as  a  provision  to  allow  the 
passage  of  a  large  mouthful,  but  are  strongly  developed  in 
front  and  serve  to  keep  the  trachea  distended.  The  most  an- 
terior of  these  rings  is  much  heavier  than  any  of  the  others 
and  is  probably  formed  by  the  fusion  of  several  of  them.  It 
is  known  as  the  cricoid  cartilage  and  is  topographically  con- 
sidered a  part  of  the  larynx.  The  tracheal  rings  must  have 
been  developed  in  some  way  from  the  tracheal  pieces  that 
segmented  off  from  the  lateral  cartilages,  but  the  manner  of 
their  formation  is  not  known.  Similar  rings  occur  in  the 
trachea  of  the  Gymnophiona,  the  rare  Order  of  subterranean 
amphibians,  but  whether  these  are  homologous  with  those  of 
the  reptiles  and  birds  has  not  yet  been  determined. 

There  is  but  little  variation  in  laryngeal  form  among  the 
representatives  of  the  Sauropsida,  and  this  in  spite  of  the  great 
differentiation  of  voice  in  the  case  of  the  birds,  since  in  these 
the  voice  is  produced,  not  by  the  larynx,  but  by  a  special  organ, 
the  syrinx,  or  lower  larynx,  situated  at  the  forking  of  the 
bronchi  and  not  found  outside  of  the  group  of  birds. 

In  mammals  a  conspicuous  addition  is  seen  in  the  thyreoid 
cartilage.  The  origin  of  this  piece  is  not  apparent  in  pla- 
cental  mammals,  in  which  it  appears  as  an  extensive  shield, 
covering  the  ventral  surface  of  the  entire  organ,  but  in  the 
more  primitive  monotremes,  instead  of  the  single  shield-like 
piece,  there  are  two  pairs  of  narrow  bars  which  from  their 
origin  and  their  similarity  to  the  more  anterior  ones,  as  well 


as  from  their  mode  of  development,  are  clearly  s 
branchial  arches,  evidently  the  2nd  and  3rd  (Fig.  8 
leaves  only  the  first  arch,  which  in  this  Order  unites 
true  hyoid  arch  to  form  the  hyoid  complex  ("  hyo 
of  human  anatomy),  to  which  it  contributes  its 
cornua,  the  thyreo-hyals.  The  cricoid  cartilage  is  n 
Sauropsida  and  is  manifestly  the  result  of  the  con 


en  to  be 
;).     This 
with  the 
id  bone  " 
posterior 
uch  as  in 
solidation 
of  certain  of  the  upper  tracheal  rings. 

The  development  of  the  lungs  is  mainly  along  the  lines  of 


THE    DIGESTIVE    AND    RESPIRATORY    SYSTEM     315 


physiological  efficiency  through  a  repeated  subdivision  of  the 
interior.  This  results  in  the  production  of  smalft  chambers, 
commonly  known  as  "  air-cells,"  more  properly  alvboli,  which 
are  connected  with  the  bronchi  by  means  of  numerous  smaller 
branches.  The  walls  of  these  alveoli  are  covered  with  a  net- 
work of  capillaries,  thus  making  them  the  ultimaie  organs 
of  respiration  to  which  all  other  parts  are  accessory.  Pri- 
marily there  are  no  cartilages  in  the  lungs  themselves,  but 
in  reptiles  they  may  be  seen  to  develop  along  the  course  of 


a 


FIG.  88.     Larynx  of  Echidna  (monotreme).     [After  GCEPPERT.] 

(a)   Ventral,      (b)   Lateral. 

St.   H,  stylo-hyal;   EH,   epi-hyal;    CH,   cerato-hyal;   BH,   basi-hyal;    Th.   H,   thyro- 
hyal;    Thy,    I,    first   thyreoid   cartilage;    Thy,    2,    second   thyreoid   cartilage. 

the  bronchi  and  invade  the  lungs;  in  mammals  the  smaller 
bronchial  tubes  are  similarly  equipped,  almost  as  far  as  the 
ultimate  branches,  although  in  the  course  downwards  the 
rings  become  less  complete  and  are  finally  reduced  to  irregu- 
lar pieces  lying  in  the  sides  of  the  tubes.  The  smallest  tubules, 
which  are  without  cartilaginous  pieces,  are  termed  bronchioli. 
In  birds  and  in  many  mammals  the  lungs  are  subdivided 
by  grooves  into  lobes,  but  in  other  cases  the  grooves  are  shal- 
low, and  the  lobes  become  hardly  more  than  slight  protuber- 


3i6  HISTORY    OF   THE    HUMAN    BODY 

ances.  Although  quite  constant  in  number  and  arrangement 
in  a  given  mammal  there  is  the  greatest  variation  of  the 
lobes  in  forms  not  closely  related ;  and  that  these  parts  are  of 
slight  physiological  importance  is  shown  by  their  complete 
absence  in  mammals  quite  unlike  structurally  and  occupying 
different  environments.  Thus  the  lungs  are  without  lobes  in 
the  Cetacea,  Sirenia,  and  some  seals,  thus  suggesting  a  modi- 
fication due  to  an  aquatic  life,  but  on  the  other  hand  the  lungs 
are  similarly  undivided  in  sloths  and  ant-eaters,  and  in  cer- 
tain rodents,  as  mice  and  squirrels.  The  left  lung  in  the 
elephant  is  also  without  lobes. 

The  development  of  a  diaphragm  in  mammals  separates  the 
general  body-cavity  into  thoracic  and  abdominal  portions  and 
cuts  off  the  pleura,  which  invests  the  lungs  and  lines  the  tho- 
racic cavity,  from  the  peritoneum,  which  stands  in  similar 
relationship  to  the  abdominal  viscera.  These  changes  cause 
some  variation  in  the  mechanism  of  breathing,  in  which  the 
diaphragm  becomes  a  powerful  accessory  organ. 


CHAPTER   VIII 
THE  VASCULAR   SYSTEM 

"  However,  if  we  consider  that  all  the  characteristics 
which  have  been  cited  are  only  differences  in  degree 
of  structure,  may  we  not  suppose  that  this  special 
condition  of  organization  of  man  has  been  gradually 
acquired  at  the  close  of  a  long  period  of  time,  with 
the  aid  of  circumstances  which  have  proved  favorable? 
What  a  subject  for  reflection  for  those  who  have  the 
courage  to  enter  into  it !  " 

LAMARCK  in  Recherches  sur  V Organization 
des  corps  vivans.  1802.  Transl.  Packard, 
1901. 

A  VASCULAR  system  of  some  sort  occurs  in  all  ccelomate 
animals,  except  in  some  reduced  parasitic  forms,  and  consists 
essentially,  of  a  cavity,  or  series  of  connected  cavities,  in  which 
a  fluid  circulates,  containing  detached  cells  of  one  or  more 
kinds.  Both  fluid  and  cells  are  concerned  in  metabolism 
and  act  as  carriers  of  material  both  to  and  from  the  various 
tissues.  In  many  Metazoa,  especially  the  smaller  and  less 
highly  organized  ones,  the  system  is  lacunar,  and  the  circu- 
lating medium,  here  often  termed  the  perivisceral  fluid,  occu- 
pies everywhere  the  irregular  spaces  between  the  organs,  and 
its  circulation  is  furthered  by  the  movements  of  these  latter 
and  of  the  entire  body;  in  other  cases  the  lacunar  system  be- 
comes reinforced,  or  largely  replaced,  by  the  formation  of 
definite  channels  in  the  form  of  branching  tubes,  through 
which  the  fluid  circulates.  Such  a  circulation  is  said  to  be 
dosed,  in  distinction  from  the  lacunar  or  open  type  first 
mentioned,  and  in  such  a  system,  deprived  as  it  is  of  the  pro- 
pelling power  insured  by  the  movement  of  external  parts,  de- 
pendence must  be  placed  upon  some  intrinsic  force  within  the 
vascular  system  itself,  and  thus  there  arise  pulsating  vessels, 

317 


318  HISTORY    OF    THE    HUMAN    BODY 

certain  localized  portions  of  the  system  of  tubes,  the  walls  of 
which  are  caused  to  dilate  and  contract  rhythmically  through 
the  development  of  a  layer  of  involuntary  muscles. 

Vertebrates  possess  the  tubular  or  closed  type  of  vascular 
system,  reinforced  by  a  few  definitely  localized  lacuna,  and 
indirectly  aided  by  the  various  serous  cavities  of  the  body  like 
the  ccelom  and  the  capsules  of  the  joints.  Both  anatomically 
and  physiologically  this  system  is  divided  into  two  subordi- 
nate systems,  hccmal  and  lymphatic,  of  which  the  first  is  the 
one  principally  emphasized,  while  the  other  bears  to  it  the 
relationship  of  an  important  auxiliary.  The  tubules  of  the 
first  system  are  divided  into  the  heart,  a  localized  pulsating 
vessel  with  enormously  hypertrophied  muscular  walls ;  arteries, 
in  which  the  current  flows  from  the  heart ;  veins,  in  which  the 
current  flows  toward  the  heart ;  and  lastly  capillaries  and  sinu- 
soids, two  forms  of  the  minute  vessels  which  extend  between 
the  arteries  and  veins  and  supply  every  tissue  of  the  body.  To 
these,  which  collectively  bear  the  name  of  blood-vessels,  there 
are  associated  a  few  definitely  bounded  lacuna,  here  spaces 
limited  by  membranes,  and  mainly  differing  from  the  rest  of 
the  system,  into  which  they  are  continued,  by  the  absence  of 
walls  of  their  own.  The  circulatory  medium  contained  in 
this  system  is  termed  blood,  and  consists  of  two  main  types 
of  cells,  the  erythrocytes  or  "  red  blood  corpuscles,"  and  the 
leucocytes  or  "  white  blood  corpuscles,"  suspended  in  a 
liquid  plasma. 

The  auxiliary  system  consists  primarily  of  lymphatic  vessels, 
which  in  distinction  from  the  veins  and  arteries  are  small 
and  thin-walled,  and  of  lymph  glands,  which  are  not  glands 
in  the  usual  sense,  but  storehouses  for  leucocytes.  With  the 
lymphatic  system  are  associated  the  serous  cavities  of  the 
body  (ccelom,  capsules  of  joints,  bursse  about  the  larger  ten- 
dons, etc.),  with  which  numerous  lymphatic  vessels  communi- 
cate so  that,  by  a  physiological  though  not  a  morphological 
right,  these  cavities  have  been  considered  as  expanded 
lymphatic  vessels.  In  the  lower  vertebrates  a  number  of 
definitely  located  pulsating  organs,  or  lymph  hearts,  further 


THE   VASCULAR    SYSTEM  319 

the  circulation  of  the  liquid  medium,  which  is  here  termed 
lymph  and  consists  of  plasma  containing  leucocytes  alone. 

These  two  latter  elements  constantly  escape  from  the  blood 
through  the  walls  of  the  capillaries  during  the  process  of 
feeding  the  tissues,  and  it  is  one  of  the  functions  of  the  lym- 
phatic system  to  collect  these  by  means  of  its  smaller  vessels 
and  eventually  to  return  them  to  the  blood.  The  other  main 
function  of  the  lymphatic  system  is  to  aid  in  the  extraction 
of  digested  foods  from  the  alimentary  canal  and  convey  them 
also  to  the  circulatory  system. 

In  no  system  of  the  body  does  the  embryonic  record  tell 
the  story  of  the  race  development  so  completely  as  in  the 
case  of  the  circulatory  system,  and  although  the  change  in 
vertebrate  history  from  water  to  air,  replacing  one  set  of 
respiratory  organs  by  another,  has  profoundly  modified  the 
blood-vessels,  yet  even  this  change  is  repeated  with  great 
fidelity  in  the  individual  life  of  each  of  the  higher  vertebrates. 
This  might  be  expected  of  the  amphibians,  in  the  most  of 
which  the  actual  change  of  external  environment  is  individually 
experienced,  yet  a  similar  metamorphosis  in  the  circulatory 
system  takes  place  in  Sauropsida  and  Mammalia,  although 
it  is  confined  to  embryonic  life. 

Since  this  is  so,  the  best  introduction  to  the  history  of  the 
circulatory  system  is  that  furnished  by  embryology,  the  early 
part  of  which  may  be  here  given  in  the  form  of  a  general 
sketch,  which,  although  not  intended  to  represent  the  details 
of  development  in  any  one  animal,  or  even  of  any  one  group, 
yet  is  based  rather  more  upon  the  development  of  the  higher 
vertebrates,  since  in  these  alone  is  the  story  complete.  In 
beginning  this  sketch  a  certain  characteristic  of  nearly  all 
vertebrate  embryos  must  be  emphasized,  since  it  is  closely 
connected  with  the  circulatory  system,  especially  in  its  earlier 
stage,  and  that  is,  the  extra-embryonal  yolk-sac,  which  de- 
velops a  set  of  blood-vessels  for  the  purpose  of  feeding  the 
embryo. 

In  this  is  seen  a  probable  reason  why  the  history  of  this 
system  is  retained  in  so  much  more  perfect  condition  than 


320  HISTORY   OF   THE    HUMAN    BODY 

are  most  of  the  others,  and  that  is,  because  this  system  is 
actively  functional  almost  from  the  beginning  of  embryonic 
life,  while  in  other  cases  the  parts  lie  passive  and  let  themselves 
gradually  assume  the  final  shape  without  contributing  anything 
to  the  functional  life  of  the  organism,  a  condition  most  favor- 
able  to  the  suppression  of  intermediate  stages. 

The  early  vertebrate  embryo,  during  its  cleavage  stages,  ap- 
pears most  frequently  as  a  circular  disc  of  rapidly  proliferating 
cells  floating  on  the  surface  of  a  spherical  or  spheroidal  yolk- 
mass  ;  and  although  at  first  these  cells  possess  sufficient  energy 
within  themselves  to  continue  development,  there  soon  comes 
a  time  at  which  they  become  dependent  upon  the  nutriment 
stored  in  the  yolk,  and  it  is  thus  one  of  the  earliest  cares  of 
the  organism  to  develop  blood-vessels  for  the  purpose  of  carry- 
ing yolk  granules  to. the  embryonic  area. 

These  blood-vessels  first  appear  as  irregularly  branching 
spaces  on  the  surface  of  the  yolk  beyond  the  limit  of  the 
definite  embryo;  these  spaces  soon  form  themselves  into  a 
capillary  net-work  and  unite  upon  each  side  of  the  embryo 
into  a  vitelline  vein. 

Within  the  embryo  a  similar  process  lays  down  the  first 
blood-vessels  and  the  entire  system  appears  as  in  Fig.  89,  A. 
The  two  vitelline  veins  unite  into  a  median  vessel,  the  future 
heart,  situated  ventrally  with  reference  to  the  embryo,  and 
immediately  back  of  the  future  gill  region.  Further  anteriorly 
the  median  vessel  divides  and  forms  two  lateral  loops,  the 
first  arterial  gill-arches,  which  continue  around  the  pharynx 
until  they  come  in  contact  with  one  another  upon  the  ventral 
side  of  the  notochord,  from  which  point  they  run  backwards, 
forming  two  aorta. 

At  a  point  a  little  posterior  to  the  entrance  into  the  embryo 
of  the  vitelline  veins,  the  aortae  pass  mainly  into  the  forma- 
tion of  two  vitelline  arteries,  which  spread  out  over  the  yolk, 
but  the  small  vessels  which  continue  into  the  posterior  end 
of  the  embryo  form  morphologically  their  real  continuation. 
During  later  development  the  posterior  aortcc  fuse  into  one 
and  increase  greatly  in  size  so  that  the  proportions  between 


1HE   VASCULAR   SYSTEM 


321 


them  and  the  vitelline  arteries  become  reversed;  and,  as  this 
part  of  the  embryo  expands  and  develops  legs,  tail,  and  pelvic 
organs,  these  latter  become  supplied  by  secondary  branches 
from  this  main  trunk.  A  similar  arterial  supply  to  the  head 
region  is  furnished  by  the  artery  which  develops  anteriorly 
from  the  arterial  gill-arch.  In  the  figure  it  appears  as  a  mere 
stump,  but  is  destined  to  become  the  carotid  artery,  which  in- 


FIG.  89.     Early  circulation  of  vertebrate  embryo. 

(A)    First  appearance  of  definite  vessels.      (B)    Later  stage,   during  the   formation 
of  gill  arteries. 

AC,  carotid  artery;   A,  dorsal   aorta;  AV ,  vitelline  artery;    VV ,  vitelline  vein;  H, 
heart. 

creases  in  size  and  the  complexity  of  its  branches  in  exact 
proportion  to  the  development  of  the  part  which  it  supplies. 
As  these  last  two  vessels,  carotid  artery  and  posterior  aorta, 
distribute  the  blood  ouside  of  the  main  channel,  a  new  set  of 
vessels  must  be  developed  to  bring  it  back  again  and  thus 
complete  the  circuit.  Those  appear  in  the  form  of  the  four 
cardinal  veins,  two  anterior  and  two  posterior  (not  shown  in 


322  HISTORY    OF   THE    HUMAN    IODY 

the  figure),  which  collect  the  blood  sent  to  the  growing  tis- 
sues of  the  embryo  by  the  arteries  and  return  it  into  the  main 
channel.  The  anterior  and  posterior  cardinals  of  each  side 
unite  opposite  the  heart  and  form  a  lateral  vessel,  the  duct  of 
Citvier  (ductus  Cuvieri),  which  enters  the  heart  from  the 
side  immediately  after  its  formation  through  the  union  of  the 
two  vitelline  veins.  In  this  system  of  vessels  is  seen  the  first 
systemic  venous  system,  the  function  of  which  is  to  collect 
from  the  body  the  blood  supplied  it  by  the  arteries  and  return 
it  to  the  heart. 

A  considerable  advance  is  seen  in  Fig.  89,  B,  which  repre- 
sents a  somewhat  older  embryo.  The  heart  has  increased  both 
in  caliber  and  in  length,  which  has  caused  it  to  assume  a  some- 
what contorted  attitude,  the  prelude  to  those  later  changes 
which  will  result  in  the  formation  of  a  compact  organ  with 
definite  compartments.  To  the  single  arterial  loop  which 
forms  the  first  arterial  arch  in  the  gill  region  others  have  been 
added  in  a  posterior  direction,  the  general  method  of  forma- 
tion being  shown  by  the  last  one,  in  this  figure  the  5th.  The 
appearance  of  limbs  has  caused  the  development  of  arteries  to 
supply  them,  subclavians  for  the  anterior,  and  iliacs  for  the 
posterior;  these  are  duplicated  by  veins  associated  with  the 
cardinal  system. 

At  about  this  stage  a  striking  change,  but  one  the  signifi- 
cance of  which  is  mainly  embryonic,  consists  in  the  develop- 
ment of  the  bag-like  allantois  with  its  accompanying  blood- 
vessels, the  allantoic  (or  umbilical)  veins  and  arteries  (see 
Fig.  17).  This  appears  indeed  in  amphibians  as  an  evagina- 
tion  from  the  ventral  wall  of  the  cloaca,  where  it  functions 
as  a  urinary  bladder,  but  here  it  never  surpasses  the  limits  of 
the  body ;  in  Sauropsida  and  Mammalia,  however,  it  develops 
into  an  enormous  extra-embryonal  organ  of  great  functional 
importance.  It  begins  in  the  form  of  a  small  sac  that  pushes 
its  way  out  from  the  embryo,  and  is  supplied  with  two  arteries 
from  the  posterior  aorta,  and  two  veins  which  enter  the  heart 
in  association  with  the  vitelline  vein,  but  it  soon  increases 
greatly  in  size,  and  its  blood-vessels  increase  proportionately. 


THE   VASCULAR    SYSTEM  323 

Ultimately,  in  Sauropsida,  animals  with  very  large  eggs  en- 
cased in  a  porous  shell,  the  allantois  comes  to  line  the  entire 
shell  and  serves  as  the  embryonal  respiratory  organ;  in  mam- 
mals it  forms  the  main  part  of  the  placenta  and  umbilical  cord, 
and  functionally  replaces  the  yolk  sac,  which  is  here  a  useless 
rudiment,  although  equipped  with  its  full  complement  of 
blood-vessels.  In  both  cases  the  allantois  is  cast  away  from 
the  embryo  at  birth,  haemorrhage  being  prevented  by  an 
atrophy  of  the  blood-vessels  at  the  point  at  which  they  leave 
the  body. 

Further  important  modifications  of  the  circulatory  system 
are  caused  by  the  development  of  liver  and  kidneys  and  by 
the  increase  in  bulk  of  the  intestine.  Owing  to  an  original 
continuity  between  the  yolk  sac  and  the  intestine,  the  veins 
from  this  latter  organ  empty  into  the  vitelline  veins,  forming 
a  compound  vein,  composed  of  intestinal  and  vitelline  branches, 
the  omphalo-mesenteric.  Of  these  the  right  one  does  not 
develop  beyond  a  certain  point,  and  the  main,  and  ultimately 
the  entire,  duty  falls  upon  the  left.  About  this  the  develop- 
ing liver  grows,  and  in  such  a  way  as  ultimately  to  include  it 
within  its  substance,  and  as  a  result  of  this  that  part  of  the 
vein  which  runs  through  the  liver  becomes  divided  into  a 
system  of  capillaries.  The  result  of  this  is  that  the  blood 
coming  from  both  yolk  and  intestine  has  no  longer  any  way 
of  getting  directly  into  the  heart  through  a  large  vessel,  but 
must  first  pass  through  the  capillary  system  of  the  liver,  and 
be  re-collected  upon  the  other  side.  From  this  stage  on  the 
single  omphalo-mesenteric  vein,  that  originally  of  the  left  side, 
becomes  known  as  the  portal  vein,  and  the  collecting  vein  upon 
the  other  side  of  the  liver,  which  brings  the  blood  from  that 
organ  into  the  heart,  forms  the  hepatic.  Throughout  this 
portion  both  of  the  original  vitelline  veins  are  preserved,  and 
it  thus  happens  that  there  are  two  hepatic  veins,  but  only  one 
portal. 

A  similar  change  is  that  inaugurated  by  the  development  of 
the  embryonic  kidney.  The  blood  comes  back  from  the  tail 
in  a  median  caudal  vein,  which,  posterior  to  the  cloaca,  divides 


324  HISTORY    OF   THE    HUMAN    BODY 

into  two  branches.  These  pass  along  the  outer  sides  of  the 
kidneys  and  are  resolved  entirely  into  a  set  of  small  branches, 
the  vena  renales  advehentes,  which  enter  these  organs  and 
break  up  into  capillaries.  From  these  the  blood  is  re- 
collected by  veins  which  emerge  from  their  inner  edges,  the 
vence  renales  revehentes,  which  unite  to  form  the  posterior 
cardinals. 

There  is  thus  formed  a  portal  system  similar  to  that  of 
the  liver,  and  called  the  renal  portal,  in  distinction  from  the 
latter,  the  hepatic  portal.  This  relationship  is  a  permanent 
one  in  fishes  and  amphibians,  but  in  the  Sauropsida  and  Mam- 
malia the  kidneys  in  which  this  portal  system  is  developed 
function  as  such  only  in  the  embryo,  and  become  eventually 
replaced  as  kidneys  by  a  new  organ  in  connection  with  which 
no  such  portal  system  becomes  developed. 

Thus  far  in  the  development  of  the  circulatory  system  all 
Classes  of  vertebrates  agree,  allowing  for  slight  differences 
in  the  relations  of  the  extra-embryonal  parts,  such  as  the  rela- 
tive development  of  the  allantois,  or  the  amount  of  yolk ;  one 
Class,  the  lowest,  that  of  fishes,  remains  permanently  at  this 
stage,  while  the  others  progressively  modify  the  fundamental 
plan.  It  may  be  well,  then,  to  consider  the  condition  in  the 
adult  selachian,  which  is  practically  that  of  the  foregoing 
sketch,  after  which  the  later  development  of  the  various  parts 
of  the  system  may  be  taken  up  one  after  another. 

The  circulatory  system  of  the  selachians  is  represented  in 
the  accompanying  diagram  (Fig.  90),  in  considering  which 
the  reader  may  begin  at  the  heart  and  trace  the  vessels  an- 
teriorly. The  heart  is  situated  very  far  forward,  immedi- 
ately behind  the  gills,  its  embryonic  position  in  higher  animals, 
and  consists  of  four  chambers  arranged  in  a  single  longi- 
tudinal row  along  the  median  line.  The  most  posterior 
of  these,  the  sinus  venosus,  is  the  receptacle  into  which  is 
brought  the  impure  blood  from  all  parts  of  the  body.  Next 
in  order,  into  which  the  blood  passes  in  succession,  are  the 
atrium,  the  ventricle,  and  the  conus  arteriosus.  This  last 
and  most  anterior  compartment  is  prolonged  into  an 
arterial  trunk  (truncus  arteriosus),  which  breaks  up  into 


THE    VASCULAR    SYSTEM 


325 


paired  lateral  branches,  the  afferent  branchial  arteries.  These 
pass  along  the  cartilaginous  gill-arches  and  supply  the  gills, 
dividing  into  very  fine  branches  for  the  purpose.  Thus  far 


FIG.  90.  Diagram  of  primitive  vertebrate  circulation,  based  on  the 
condition  found  in  selachians. 

s,  sinus  venosus;  t,  atrium;  v,  ventricle;  x,  conus  arteriosus;  br,  branchial  arteries: 
ad,  carotid  artery;  aoa,  aortic  arch;  aod,  dorsal  aorta;  ce,  coeliac  axis,  consisting  of 
(m)  mesenteric,  and  hepatic  and  splenic  (unmarked)  branches;  can,  caudal  arteries 
and  veins;  it,  iliac  arteries  and  veins;  sb,  subclavian  arteries  and  veins;  ra,  renales 
advehentes;  rr,  renales  revehentes;  I,  lateral  vein;  cp,  posterior  cardinal  vein;  ca,  an- 
terior cardinal  vein;  p,  hepatic  portal  vein;  h,  hepatic  vein. 

the  blood  is  impure,  in  the  state  in  which  it  was  received  from 
the  body,  but  at  this  point  there  intervenes  a  system  of  capil- 


326  HISTORY   OF    THE    HUMAN    BODY 

laries,  in  which  the  exchange  of  respiratory  gases  takes  place, 
and  when  it  is  re-collected  into  the  efferent  branchial  arteries, 
corresponding  in  number  to  the  afferent  branchials,  the  blood 
has  become  aerated.  These  latter  arteries  converge  to  the 
median  line,  where  they  unite  to  form  a  median  aorta,  which 
lies  upon  the  ventral  side  of  the  vertebral  centra,  and  gives 
off  the  main  arteries  of  the  body.  Before  the  arches  of  the 
two  sides  unite  they  give  off  the  carotid  arteries,  which  supply 
the  head  and  brain ;  and  then,  not  far  from  the  point  of  union, 
the  subclavians,  to  the  anterior  paired  limbs  (pectoral  fins). 
Lower  down  appear  branches  that  supply  the  body  walls  and 
the  viscera;  and  the  posterior  paired  limbs  (ventral  fins)  are 
supplied  by  the  iliacs.  As  these  branches  are  given  off,  the 
aorta  diminishes  in  size  and  terminates  at  the  end  of  the  tail 
as  a  mere  thread,  protected  throughout  the  caudal  region  by 
the  haemal  arches  of  the  vertebrae. 

The  entire  body  is  thus  supplied  with  aerated  blood  from 
a  single  main  channel  with  its  branches,  but  on  its  return  its 
course  is  not  so  simple,  and  involves  three  distinct  venous 
systems  connected  with  one  another  by  capillaries.  The  first 
of  these  consists  of  four  great  longitudinal  veins,  the  two 
anterior  and  the  two  posterior  cardinals,  which  collect  the 
blood  from  the  head,  the  anterior  fins,  and  the  walls  of  the 
trunk.  As  in  the  embryological  sketch,  the  anterior  and  pos- 
terior cardinal  veins  of  each  side  unite  into  a  ductus  Cuvieri, 
which  enters  the  sinus  venosus.  Associated  with  the  posterior 
cardinals  are  the  two  large  lateral  veins  which  lie  in  the  body 
wall  and  were  perhaps  originally  situated  along  the  bases  of 
the  lateral  fin-folds,  from  which  the  paired  limbs  have  been 
derived.  They  arise  as  very  small  vessels  along  the  sides  of 
the  tail  and  enlarge  rapidly  as  they  proceed  anteriorly  through 
the  assumption  of  tributary  branches  from  each  somite.  In  the 
cloacal  region  the  two  lateral  veins  communicate  by  numerous 
anastomosing  branches,  forming  a  cloacal  plexus  (represented 
in  the  diagram  by  a  single  vein),  and  receive  the  iliac  veins 
from  the  posterior  fins.  Anterior  to  this  they  still  receive  meta- 
meric  contributions  from  each  somite  and  finally  empty 


THE   VASCULAR    SYSTEM  327 

into  the  posterior  cardinals  near  their  fusion  with  the  an- 
terior ones  to  form  the  ductus  Cuvieri.  The  subdavian  vein 
from  the  anterior  fin  enters  either  the  lateral  vein  or  the 
posterior  cardinal  near  the  entrance  of  the  latter.  In  the 
former  case,  which  may  be  considered  the  more  primitive,  we 
have  the  suggestion  of  the  early  relation  of  the  lateral 
vein  to  the  fin-fold,  for  this  condition  suggests  strongly  a 
primitive  one  in  which  the  lateral  vein  received  a  branch  from 
each  metameric  element  of  the  fin-fold.  When  the  definite 
limbs  were  established  by  the  hypertrophy  of  an  anterior  and 
posterior  region  and  the  loss  of  the  intermediate  portion,  the 
veins  corresponding  to  the  regions  retained  became  large  and 
important,  while  the  rest  were  somewhat  reduced.  To  ac- 
count for  the  retention  of  a  single  vein  for  each  appendage, 
rather  than  one  from  each  somite  represented,  one  may  sup- 
pose either  the  retention  of  one  and  the  loss  of  the  others,  or 
the  fusion  of  several.  Since,  in  the  pelvic  fin  of  the  skate, 
there  are,  in  addition  to  the  principal  iliac  vein,  one  or  two 
small  vessels  which  open  independently  into  the  lateral  vein, 
the  former  alternative  is  the  more  probable. 

The  second  system  begins  by  a  median  caudal  vein,  which 
starts  at  the  tip  of  the  tail  and  runs  within  the  haemal  arches, 
upon  the  ventral  side  of  the  aorta.  When  near  the  cloaca 
this  vein  divides  into  two  lateral  branches,  which  run  along 
the  lateral  margins  of  the  long  and  narrow  kidneys,  and  give 
off  to  these  organs  numerous  lateral  branches,  the  vena  renales 
advehentes.  These  break  up  into  a  capillary  system  within  the 
substance  of  the  kidneys  and  form  the  renal  portal  system. 
'From  this  capillary  net-work  the  blood  is  collected  along  the 
medial  margin  of  the  kidneys  by  numerous  vena  renales 
revehentes,  the  union  of  which  into  a  common  trunk  forms 
the  origin  of  each  posterior  cardinal. 

The  third,  or  hepatic  portal  system,  is  exactly  as  given  in  the 
embryological  sketch.  It  collects  the  blood  from  the  intestines 
and  stomach  into  a  common  trunk,  the  portal  vein,  which  enters 
the  liver  upon  its  dorsal  side  and  becomes  resolved  into  capil- 
laries, as  in  the  former  case.  From  this  organ  the  blood  is  re- 


328  HISTORY   OF   THE    HUMAN    BODY 

collected  by  one  or  more  hepatic  veins,  lying  more  on  the  ven- 
tral side  of  the  liver,  and  is  emptied  into  the  sinus  venosus. 
Thus  all  the  impure  blood,  through  one  channel  or  another, 
finds  its  way  into  this  most  posterior  chamber  of  the  heart, 
from  which  it  passes  in  succession  through  atrium,  ventricle, 
and  conus  arteriosus,  and  finally  into  the  gills,  where  it  be- 
comes aerated. 

It  thus  happens  that  the  heart  contains  only  impure  or 
venous  blood,  since  it  is  not  purified  until  it  reaches  the  gills, 
which  suggests  that  the  terms  "  arterial "  and  "  venous,"  as 
applied  to  pure  and  impure  blood  respectively,  are  not  applica- 
ble in  the  case  of  the  lower  vertebrates,  and  are  much  better 
dropped,  since  they  are  often  misleading.  Furthermore,  in 
Amphibia  and  Reptilia  these  two  kinds  of  blood  are  not  sharply 
defined,  since  both  sorts  are  often  allowed  to  mingle,  forming 
a  mixed  blood  of  varying  degrees  of  purity.  All  confusion  on 
this  point,  however,  may  be  avoided  if  the  terms  artery  and 
vein  and  their  corresponding  adjectives  are  used  in  their  an- 
atomical sense  only,  arteries  being,  as  previously  defined,  those 
vessels  in  which  the  blood  flows  from  the  heart,  and  veins  those 
in  which  the  blood  flows  towards  the  heart.  The  physiological 
distinction  which  designates  pure  blood  as  arterial  and  impure 
blood  as  venous  is  taken  from  its  condition  in  the  two  sets  of 
vessels  in  birds  and  mammals,  and  even  here  in  the  case  of 
the  pulmonary  system  the  conditions  are  reversed  and  physio- 
logically arterial  blood  flows  in  the  veins,  and  physiologically 
venous  blood  in  the  arteries. 

The  history  of  the  arterial  arches  is  shown  in  synoptical 
form  by  the  accompanying  series  of  diagrams  (Fig.  91 ),  which 
present  the  facts  as  deduced  from  the  combined  study  of  both 
the  adult  anatomy  and  embryological  development  of  repre- 
sentatives of  each  Class  of  vertebrates.  The  diagrams  repre- 
sent the  adult  conditions  in  each  case,  the  relationship  being 
morphologically  interpreted  by  the  help  of  the  development. 

There  are  typically  six  pairs  of  arterial  arches,  which  lie 
along  the  sides  of  the  pharynx  and  extend  from  a  ventral  vessel 
that  proceeds  directly  from  the  heart  to  a  dorsal  one  that  col- 


THE    VASCULAR    SYSTEM 


lects  the  blood  from  the 
arches  and  conveys  it  to 
all  parts  of  the  body,  the 
ventral  and  dorsal  aortse 
respectively  (Fig.  91,  a),  f 
In  all  the  diagrams  the  • 
parts  of  both  sides  are 
shown,  viewed  ventrally 
and  flattened  out  so  that 
the  ventral  aorta  lies  in 
the  middle  and  the  dorsal 
aortse  converge  from  the 
outer  sides.  In  selach- 
ians (Fig.  91,  b)  five  of 
these  arches  are  present 
and  functional;  each  arch 
is  divided  into  an  afferent 
and  an  efferent  branch, 
between  which  respiration 
is  effected  by  means  of 
capillaries  spread  out  over 
soft  endodermic  gills. 
From  the  anterior  portion 
of  the  efferent  system  the 
carotids  are  given  off, 
vessels  which  include  the 
only  remnants  of  the  first 
arterial  arches. 
/fin  the  urodelous  am- 
phibians (Fig.  91,  c)  the 
first  two  arterial  arches 
disappear  in  the  embryo, 
leaving  four  functional 
arches.  Of  these  arch 
III  unites  with  remnants 
of  I  and  II  to  form  the 
carotids,  IV  and  V  on 


FIG.  91.  Diagrams  showing  modifi- 
cations in  the  arterial  arches  of  Ver- 
tebrates. 

(a)  Typical,  embryonic,  (b)  Fishes.  (c) 
Amphibians.  (d)  Reptiles.  (e)  Birds.  (f) 
Mammals. 

/,  //.  ///,  IV,  V,  VI,  arterial  arches;  of, 
Art.  carotis  dextra;  cs.  Art.  carotis  sinistra; 
sd,  Art.  subclavia  dextra;  ss,  Art.  subclavia 
sinistra;  ad,  Aorta  dextra;  ay,  Aorta  sinistra; 
bd.  Ductus  Botalli  dexter;  bs,  Ductus  Botalli 
sinister;  pd,  Art.  pulmonalis  dextra;  pst 
Art.  pulmonalis  sinistra. 


330  HISTORY   OF   THE    HUMAN    BODY 

both  sides  form  complete  arches  and  unite  to  form  the  dorsal 
aorta,  and  VI  becomes  the  pulmonary  (here  the  puhno-cutane- 
ous).  Of  the  two  aortic  arches  IV  is  the  principal  one  and  V 
is  a  subordinate,  and  is  of  such  slight  functional  importance 
that  in  the  higher  Classes  it  is  destined  to  disappear  altogether. 
These  arches  are  usually  continuous,  and  are  not  as  a  rule 
interrupted  in  the  midst  by  the  interposition  of  respiratory 
capillaries  as  in  fishes;  in  larval  urodeles,  however,  and  in  a 
few  adult  forms,  the  perennibranchs,  a  branch  from  the  ven- 
itral  side  of  the  arch  supplies  the  external  gill-bushes  with 
capillaries,  from  which  a  collecting  branch  returns  the  blood 
to  the  arch  at  its  dorsal  end.  When  such  a  gill-bush  is  of  much 
functional  importance  these  lateral  branches  are  large,  and  in 
extreme  cases  it  is  possible  that  practically  all  the  blood  of  a 
given  arch  may  pass  through  these  indirect  channels.  In  most 
cases  the  external  gills,  and  with  them  their  supplying 
branches,  disappear  at  the  expiration  of  larval  life,  and  the 
arches  form  continuous  vessels,  as  in  higher  forms.  The  sixth 
arch  is  in  the  larva  a  complete  one,  and  joins  the  dorsal  aorta, 
as  do  the  two  preceding ;  with  the  development  of  the  lungs 
and  the  integumental  respiration  a  small  branch,  which  arises 
from  this  arch  near  the  middle,  becomes  engaged  in  supplying 
the  lungs  and  skin,  and  increases  so  much  in  size  that  it 
ultimately  transmits  all  of  the  blood  that  enters  the  arch, 
leaving  the  distal  half  of  the  arch  without  employment.  This 
part  then  closes  its  lumen  and  is  retained  as  a  connecting 
band,  the  ligamentum  arteriosum  [Botalli],  extending  along 
its  old  path  between  the  pulmonary  artery  and  the  dorsal 
aorta.  A  similar  ligament,  or  in  many  cases  a  small  perviotis 
artery,  is  also  retained  between  the  carotid  arch  and  the  main 
aortic  arch  (III  and  IV). 

In  reptiles  (Fig.  91,  d)  the  metamorphosis  of  the  arterial 
system  is  pushed  back  into  embryonic  life,  and,  from  this  point 
on,  no  longer  appears  after  birth.  In  other  words,  the  transi- 
tion from  water  to  land,  an  historic  scene  actually  enacted 
during  the  post-natal  existence  of  amphibians  in  the  form  of 
the  metamorphosis,  with  all  the  changes  involved,  not  only  in 


THE   VASCULAR   SYSTEM  331 

the  circulatory,  but  in  other  systems  as  well,  is  pushed  back 
among  the  stages  that  are  recapitulated  in  the  embryo;  there 
is  a  metamorphosis  in  reptiles  and  mammals  just  as  truly  as 
in  the  case  of  amphibians,  but  it  is  embryonic.  Here,  as  in 
amphibians,  arch  III,  with  rudiments  of  I  and  II,  forms  the 
carotids  and  its  connection  with  arch  IV  disappears.  This 
latter  becomes  the  aortic  arch,  and  is  retained  on  each  side, 
as  right  and  left  arches,  the  two  uniting  dorsally  and  back  of 
the  heart  to  form  the  main  aorta.  Arch  V,  which  in  am- 
phibians is  practically  superfluous,  is  given  up  in  reptiles, 
and  from  this  point  on  is  seen  no  more,  save  in  the  em- 
bryo, where  it  often  appears  as  a  rudiment.  A  pulmonary 
artery  develops  from  the  sixth  arch  of  each  side,  as  in  am- 
phibians, leaving  a  right  and  left  ligamentum  arteriosum  \_Bo- 
talli].  The  subclavian  arteries,  that  supply  the  fore-limbs, 
which  in  most  fishes  and  amphibians  arise  from  the  dorsal 
aorta  after  the  union  of  the  two  lateral  arches,  possess  a  more 
anterior  origin  and  arise  from  the  right  aortic  arch.  Croco- 
diles and  turtles  present  an  exception  to  this,  and  in  these, 
as  in  birds,  the  subclavians  arise  from  the  base  of  the  carotids, 
an  origin  so  radically  different  as  to  lead  morphologists  to  be- 
lieve that  these  vessels  are  not  the  subclavians  at  all,  but  are 
secondarily  developed  arteries  (subclavice  secundaric?)  which 
have  functionally  replaced  the  true  subclavians. 

In  birds  and  mammals  (Fig.  91,  e  and  f)  but  a  single 
aortic  arch  comes  to  development ;  in  birds  this  is  the  one  on 
the  right  side,  in  mammals  the  one  on  the  left,  a  convincing 
proof,  if  proof  were  wanting,  of  the  independent  development 
of  these  two  Classes.  There  is  thus,  in  each  case,  but  one 
ligamentum  arteriosum,  connecting  the  pulmonary  and  aortic 
arches.  In  the  mammalian  fetus,  in  which  pulmonary  respira- 
tion is  not  assumed  till  the  moment  of  birth,  this  vessel  is 
functional  and  is  known  as  the  ductus  arteriosus  [Botalli]. 
It  is  still  pervious  at  birth,  but  the  lumen  closes  within  a  few 
days  by  the  rapid  thickening  of  the  wall  of  the  vessel.  There 
is  here  to  be  noted  an  important  difference  also  in  the  sub- 
clavians; in  birds,  as  in  the  turtles,  these  vessels  are  repre- 


332  HISTORY    OF    THE    HUMAN    BODY 

sented  by  sub-clavi<z  secundarice,  branches  of  the  carotids,  but 
in  mammals  they  are  the  primary  vessels,  homologous  with 
those  of  typical  reptiles.  There  is,  indeed,  a  new  morpho- 
logical distinction  between  those  of  the  two  sides,  for  while 
the  left  one,  that  of  the  side  which  furnishes  the  aortic  arch, 
arises  from  this  arch,  that  of  the  right  is  the  equivalent,  not 
only  of  that  of  the  left  side,  but  of  the  fourth  arch  as  well. 
It  is  this  relationship  which  causes  the  intimate  association 
upon  the  right  side  between  the  subclavian  and  carotid,  and 
the  short  common  trunk,  arteria  anonyma  [innominata],  is 
thus  the  region  once  common  to  arches  III  and  IV.  Thus, 
while  these  vessels  on  the  right  side  are  superficially  similar 
in  both  birds  and  mammals,  they  are  morphologically  totally 
different.  In  the  former  the  "  subclavian  "  is  a  secondarily 
formed  branch  of  the  carotid,  with  the  true  subclavian  prob- 
ably suppressed ;  in  the  latter  the  "  subclavian  "  is  the  fourth 
arch  plus  the  true  subclavian  that  once  branched  from  the 
point  where  this  arch  joined  an  aorta,  as  on  the  other  side. 

In  this  relationship  is  seen  also  the  explanation  of  the  curious 
asymmetry  in  the  origin  of  the  human  carotids  and  subclavians, 
a  condition  which  is  undoubtedly  a  primitive  one.  This  be- 
comes more  complicated  in  many  other  more  specialized  mam- 
mals by  various  secondary  approximations  and  fusions.  Thus, 
in  the  Carnivora  the  left  carotid  fuses  near  its  base  with  the 
other  and  produces  the  phenomenon  of  three  of  the  four  ar- 
teries in  question  arising  from  a  common  stump,  while  the  left 
subclavian  is  alone  distinct :  in  ruminants  this  latter  also  shifts 
its  position  forward  and  fuses  with  the  others,  so  that  a  single 
median  trunk  arises  from  the  crest  of  the  arch.  This  gives 
off  first  the  two  subclavians,  and  then,  after  continuing  for- 
ward a  little,  divides  into  the  two  carotids. 

Ontogenetically  the  most  anterior  of  the  six  arterial  arches 
is  the  first  to  appear,  and  this,  with  the  ventral  and  dorsal 
aortae,  forms  a  lateral  loop  directed  forwards.  The  other 
arches  are  successively  added  through  the  formation  along  the 
course  of  the  aortae  of  buds  or  sprouts  that  meet  and  join.  In 
fishes  all  but  the  first  of  the  arches  are  retained,  but  in  higher 


THE   VASCULAR   SYSTEM  333 

forms,  where  the  first  two  become  lost,  these  begin  to  degen- 
erate before  the  more  posterior  ones  appear.  In  the  mam- 
malian embryo  there  are  in  the  appearance  of  these  arches 
two  important  points  to  notice ;  first,  the  successive  supremacy 
in  size  and  function  of  each  arch  down  to  the  fourth,  and, 
second,  the  extremely  rudimentary  condition  of  arch  V, 
amounting  in  some  cases  to  a  complete  suppression.  This  is 
shown  in  the  rabbit  embryo  in  the  four  states  given  in  Fig.  92. 
In  a  arch  I  is  the  principal,  or,  in  fact,  the  only  functional 
one,  and  II  and  III  are  forming  from  approximated  dorsal 
and  ventral  buds.  In  b  arch  I  has  become  disintegrated, 
while  the  chief  function  is  assumed  by  arch  II:  the  ventral 
bud  of  arch  IV  is  also  seen.  In  c  both  first  and  second  arches 
are  lost  and  their  remnants  appear  in  part  as  continuations  of 
dorsal  and  ventral  aortae  and  in  part  as  stumps  of  vessels  from 
which  important  branches  of  the  carotid  system  are  to  be  de- 
veloped. Arch  V  appears  at  about  its  maximum  here  and  in 
the  next  figure,  and  the  ventral  bud  of  arch  VI  has  become 
well  developed.  In  d  arch  IV,  to  be  later  the  permanent 
aortic  arch,  is  assuming  good  proportions,  and  arch  VI,  the 
future  pulmonary  arch,  is  completed.  From  the  dorsal  side 
of  the  dorsal  aorta  appear  the  beginnings  of  certain  transitory 
arteries  that  correspond  to  the  head  somites. 

The  complete  ontogenetic  history  of  the  carotid  system  in 
mammals  shows  a  curious  shifting  of  branches  from  one  source 
to  another,  and  the  development  and  decay  of  transitory  ele- 
ments. In  this  the  remnants  of  arches  I  and  II,  long  sup- 
posed to  be  lost,  play  a  prominent  part,  and  unite  with  the  main 
carotid  arch  (III)  in  the  formation  of  the  system.  This 
history,  together  with  that  of  the  other  arterial  arches,  is  seen 
in  the  accompanying  set  of  diagrams  (Figs.  93  and  94),  based, 
with  the  exception  of  the  last  one,  upon  the  embryology  of  the 
rat.  In  a  is  seen  the  first  arch  [Cf.  Fig.  89,  a]f  which  starts 
from 'the  conus  arteriosus  (ca),  proceeds  forward,  and  re- 
turns as  the  dorsal  aorta  (ad),  giving  forth  a  carotid,  the 
arteria  carotis  cerebralis  (ccb),  just  before  returning.  The 
ventral  bud  of  arch  II  has  also  appeared.  In  b  -arch  II  has 


334 


HISTORY   OF   THE    HUMAN    BODY 


THE   VASCULAR    SYSTEM  335 

become  completed  and  arch  III  is  forming  from  ventral  and 
dorsal  buds,  and  in  c,  arches  I,  II,  and  III  are  complete,  with 
a  ventral  bud  forming  for  arch  IV.  Arches  IV  and  VI  are 
both  formed  in  d,  with  several  "  islands  "  in  the  former,  which 
probably  have  no  especial  significance.  Arch  I  has  broken, 
leaving  dorsal  and  ventral  stumps. 

From  the  dorsal  aorta  appear  segmental  arteries  (s)  which 
soon  disappear;  of  these  the  lowest  is  the  hypo  glossal  (hy). 
Below  this  are  other  segmental  arteries,  of  which  the  first 
cervical  (ec)  is  figured  here.  Stage  e  shows  but  little  change 
save  in  proportions  and  the  loss  of  segmental  arteries.  A 
pulmonary  artery  appears,  arising  from  arch  VI.  The  human 
embryo  at  about  this  stage  shows  a  well-developed  arch  V. 
The  segmental  arteries  of  the  head  have  disappeared.  In  f 
arch  II  has  also  broken  through,  leaving  stumps,  and  of  these 
the  ventral  becomes  closely  associated  with  that  of  arch  I,  and 
both  are  borne  by  the  anterior  end  of  the  ventral  aorta  (az>), 
a  part  destined  to  play  an  important  role  later  on.  Arch  III 
has  become  large;  arch  IV  is  very  large,  and  a  rudiment  of 
arch  V  has  appeared.  The  dorsal  stump  of  I  has  divided 
into  two  branches,  the  maxillaris  (ms),  which  goes  to  the  de- 
veloping upper  jaw,  and  the  mandibularis  (md),  which  be- 
comes distributed  to  the  lower  jaw. 

In  stage  g  the  maxillary  artery  just  mentioned  has  divided 
again  into  a  supra-orbital  (o)  and  an  infra-orbital  (t),  thus 
giving  three  terminal  branches  of  the  dorsal  stump  of  arch  I. 
From  the  free  end  of  the  ventral  aorta  (av)  appears  a  branch 
that  goes  to  the  tongue-anlage,  the  lingualis  (/).  The  point 
especially  to  be  noted  here  is  that  of  the  two  buds,  (x)  and 
(y),  which  arise  from  the  dorsal  stumps  of  arches  I  and  II, 
respectively,  and  grow  toward  one  another.  The  formation  of 
the  arteria  vertebralis  cerebralis  (vc)  by  the  union  of  the  hypo- 
glossal  and  first  cervical  arteries  with  one  from  much  further 
forward  is  also  to  be  noticed,  but  is  without  special  interest. 
In  stage  h  the  buds  (x)  and  (y)  have  united  the  dorsal  buds 
of  I  and  II,  and  the  significance  of  this  step  is  seen  by  compar- 
ing this  with  stage  t,  for  here  the  portion  connecting  the  com- 


336 


HISTORY   OF   THE    HUMAN    BODY 


FIG.  93.  Development  of  arterial  arches  in  Rat  embryo.  [After 
TANDLER.] 

I,  II,  III, .  IV,  V,  VI,  represent  the  respective  arches  or  their  rudiments;  a, 
conus  arteriosus;  ad,  dorsal  aorta;  av,  ventral  aorta  (truncus  arteriosus) ;  ccb,  art. 
carotis  cerebralis;  p,  arteria  pulmonalis;  ec,  first  cervical  artery;  s,  segmental  arteries, 
hy,  hypoglossal  artery;  ms,  maxillary  artery;  md,  mandibular  artery;  o,  supra- 
orbital  artery;  i,  infra-orbital  artery;  /,  lingual  artery;  vc,  art.  vertebralis  cere- 
bralis; x,  y,  stumps  which  ultimately  join,  and  form  the  stapedial  artery,  (st.) 


THE   VASCULAR    SYSTEM 


337 


mon  origin  of  the  supra-  and  infra-orbital  and  mandibular 
arteries  has  become  lost  and  their  source  of  supply  has  become 
transferred  to  the  dorsal  stump  of  II.  The  artery  thus  formed 
penetrates  the  mass  of  cells  destined  to  become  the  stapes  and 
forms  the  foramen  characteristic  of  this  bone  in  the  higher 


FIG.  94.     (h)-(l),  Continuation  of  the  series  given  in  Fig.  93. 
Ultimate  condition  in  Man,  for  comparison  with   (1).     [All  figures  after 
TANDLER.] 

xy,  the  artery  formed  by  the  union  of  x,  and  y,  in  the  previous  figure;  cc, 
common  carotid  artery;  ce,  external  carotid  artery;  ci,  internal  carotid  ?rtery;  st, 
stapedial  artery;  n,  anastomosing  branch  between  the  external  carotid  and  mandib- 
ular arteries.  The  other  abbreviations  are  given  under  FIG.  93  or  are  explained 
in  the  text. 

Mammalia.  In  the  monotremes,  where  this  action  does  not 
take  place,  the  bone  is  columnar,  and  without  a  foramen.  From 
now  on  the  artery  formed  by  the  dorsal  stump  of  arch  II,  the 
anastomosing  branch  (xy),  and  a  bit  of  the  dorsal  stump  of 
arch  I,  becomes  called  the  stapedialis  (st)f  through  its  relation- 


338  HISTORY   OF    THE   HUMAN    BODY 

ship  to  the  stapes.  How  it  comes  to  bear  the  three  important 
branches  of  the  later  external  carotid  has  been  already  seen. 

Between  stages  h  and  i  a  second  important  change  has 
been  inaugurated  in  the  reduction  of  that  part  of  the  dorsal 
aorta  which  connects  arches  III  and  IV.  This  finally  effects 
a  complete  separation  of  the  two  arches  in  this  place,  and 
causes  the  third  arch  to  become  a  common  carotid  artery  (cc) 
which  divides  into  two  branches,  an  external  carotid  (ce) 
which  was  formerly  the  anterior  part  of  the  ventral  aorta  plus 
the  ventral  stumps  of  arches  I  and  II,  and  an  internal  carotid 
(a),  the  main  third  arch  plus  the  original  arteria  carotis 
cerebralis. 

One  more  change  in  relationship  is  to  be  effected,  and  that 
is  inaugurated  through  the  growth  of  another  anastomotic 
branch  (n  in  stage  i)  which  enters  the  side  of  the  mandibularis 
(or,  perhaps,  the  continuation  of  the  stapedialis)  and  forms  a 
complete  circuit,  as  in  stage  k.  From  this  point  on  the  history 
differs  in  the  rat  and  in  Man,  as  is  indicated  by  the  two 
arrows,  with  their  respective  designations.  In  the  rat  the  cir- 
cuit breaks  at  the  point  between  the  infra-orbital  and  the  man- 
dibular,  and  in  Man  at  a  point  above  the  supra-orbital.  The 
two  results  of  these  are  seen  in  diagrams  /  and  n,  which  rep- 
resent the  adult  condition  of  this  detail  in  the  rat  and  in  Man, 
respectively.  In  the  former  (/)  the  stapedial  artery,  a  branch 
of  the  internal  carotid,  bears  both  supra-  and  infra-orbital 
arteries,  while  the  external  carotid  becomes  continued  mainly 
into  the  mandibular.  In  the  latter  the  external  carotid  bears 
all  three  of  the  branches  in  question,  while  the  stapedial  ar- 
tery, being  of  no  further  use,  disappears,  and  leaves  in  the 
stapes  the  hole  through  which  it  formerly  ran,  thus  account- 
ing for  the  particularly  curious  shape  of  this  little  bone,  which 
attracted  the  attention  of  the  early  anatomists,  but  for  which 
they  had  no  explanation.  In  considering  the  details  of  the  de- 
velopment of  any  part  of  the  circulatory  system,  the  process 
is  seen  to  be  a  metamorphosis,  correlated  with  the  changes 
in  the  parts  supplied  by  the  blood-vessels  under  consideration. 
Such  a  metamorphosis  is  like  the  changes  in  the  roads  and 


THE   VASCULAR    SYSTEM  339 

paths  of  a  given  district,  due  to  a  shifting  of  the  centers  of 
population,  and  the  development  or  decay  of  any  points  of 
human  interest.  Changes  like  these  set  the  traffic  now  over 
one,  now  over  another,  series  of  roads,  which  increase  or  de- 
crease in  width  and  degree  of  development  in  exact  propor- 
tion to  this  use,  certain  ones  becoming  highways  and  others 
lanes,  solely  through  the  functional  importance  of  the  locali- 
ties which  they  connect.  Even  the  atrophied  rudiments  have 
their  counterpart  in  the  ancient  roads,  entirely  overgrown  and 
lost  to  all  save  the  antiquary. 

The  branches  of  the  aorta  posterior  to  the  arterial  gill- 
arches  and  their  derivatives  are  sufficiently  similar  in  all  ver- 
tebrates to  be  easily  recognized,  but  it  may  be  said  in  general " 
that,  as  is  the  case  with  other  systems,  these  branches  show 
many  more  indications  of  metameric  arrangement  in  the  lower 
vertebrates,  and  are  accordingly  more  numerous.  Instances 
of  this  are  seen  in  the  numerous  lateral  and  dorsal  branches 
which  supply  the  muscles  of  the  body  wall  and  are  segmen- 
tally  arranged  in  fishes  and  amphibians,  while  in  higher  forms 
their  number  is  much  reduced,  forming  the  intercostal  and 
lumbar  arteries.  It  is  again  strikingly  shown  in  the  mesen- 
teric  arteries  which,  in  lower  forms,  are  very  numerous  and 
suggest  a  metameric  series,  while  in  higher  forms  they  are 
collected  at  their  origin  into  a  common  trunk  (Fig.  95). 

The  relative  size  of  the  various  branches  varies  directly 
with  that  of  the  parts  which  they  supply,  a  fact  especially 
noticeable  in  the  case  of  the  subclavian  and  iliac  arteries,  which 
are  small  and  unimportant  in  fishes,  with  small  lateral  fins,  but 
which  become  excessively  developed  in  connection  with  the 
hypertrophy  of  one  or  both  pairs  of  limbs.  The  caudal  aorta, 
like  the  other  elements  of  the  tail,  retains  its  primitive  charac- 
ter and  gives  off  metamerically  arranged  branches  in  the  case 
of  well-developed  tails,  in  which  the  other  parts  are  sufficiently 
emphasized  to  allow  it.  In  Man  the  caudal  artery  becomes 
reduced  to  the  insignificant  arteria  sacralis  media,  in  which  the 
earlier  anatomists  failed  to  see  the  continuation  of  the  aorta. 
This  is  in  part  due,  however,  to  the  enormous  development 


340 


HISTORY   OF   THE    HUMAN    BODY 


of  the  legs  correlated  with  the  erect  position,  which  has  de- 
veloped the  iliac  arteries  out  of  all  proportion,  giving  the  er- 
roneous but  inevitable  impression  that  these  latter  arteries 
form  the  real  continuation  of  the  aorta,  which  becomes  bi- 
furcated, and  that  the  arteria  sacralis  media  is  an  unimportant 
median  branch  arising  from  the  point  of  bifurcation  and  sup- 
plying the  coccygeal  region. 

In  the  adult  selachians,  which  in  their  venous  system  rep- 
resent practically  the  starting  point  of  the  history  so  far  as 
vertebrates  are  concerned,  the  two  sets  of  cardinal  veins,  an- 


a 


FIG.  95.     Metamerism  in  the  mesenteric  arteries  of  Amphibia.     [After 
KLAATSCH.] 

(a)    Siren,      (b)    Necturus.      (c)    Cryptobranchus.      (d)    Cryptobranchus    (a   second 
specimen).       (e)    Anura. 

terior  and  posterior,  are  in  control  of  the  venous  blood,  except 
that  from  the  alimentary  canal,  and  return  it  from  all  parts 
of  the  body  to  the  sinus  venosus.  However,  during  the  em- 
bryological  development  of  these  animals  one  catches  glimpses 
of  a  still  earlier  systemic  vein,  the  sub-intestinal,  which,  here 
embryonic  and  transitory,  must  have  preceded  the  cardinal  sys- 
tem historically,  and  have  been  totally  replaced  by  the  latter 
before  the  advent  of  true  vertebrates  as  we  now  know  them. 
It  appears  soon  after  the  establishment  of  the  two  yolk  veins, 
always  for  practical  reasons  the  first  to  appear  in  vertebrate 
embryos,  and  extends  from  the  left  yolk  vein,  from  which  it 


THE   VASCULAR   SYSTEM  341 

arises,  to  the  tip  of  the  tail,  lying  in  the  median  line,  just  ven- 
tral to  the  intestine.  At  the  very  first  it  consists  of  a  pair  of 
fine  vessels  running  very  near  one  another,  but  these  soon 
coalesce  into  a  single  median  vessel,  much  as  in  the  case  of  the 
aorta.  At  the  level  of  the  cloacal  opening  the  two  original 
elements  remain  distinct,  and  run  along  the  sides  of  the  in- 
testine, but  fuse  again  posterior  to  it,  forming  a  loop  or 
ring. 

Previous  to  this  the  cardinal  system  has  begun  its  develop- 
ment in  the  form  of  minute  vessels  which  grow  out  from  the 
sides  of  the  sinus  venosus,  and  as  they  extend  farther  and  be-j 
come  of  larger  size  the  free  ends  of  the  posterior  cardinals 
form  several  anastomoses  with  the  subintestinal  vein  anterior 
to  the  cloacal  ring  and  at  the  place  about  which  the  kidneys 
(mesonephros)  are  to  develop.  This  connection  furnishes  two 
large  lateral  channels  for  the  blood  from  the  subintestinal 
system,  a  change  which  has  two  direct  results,  first,  the  gradual 
usurpation  of  function  of  that  part  of  the  subintestinal  vein 
which  lies  anterior  to  the  anastomoses,  a  relationship  that  leads 
to  its  ultimate  disappearance,  and  second,  the  retention  of  the 
part  posterior  to  the  connection  as  the  caudal  vein,  now  become 
a  part  of  the  cardinal  system.  At  the  point  where  the  original 
anastomoses  occur,  the  development  of  the  kidneys  causes  the 
formation  of  a  rich  capillary  net-work,  a  process  which  ends  in 
the  establishment  of  the  renal  portal  system  with  caudal  veins 
for  conveying  the  blood  to  the  kidneys  and  posterior  cardinals 
for  re-collecting  it  and  conveying  it  to  the  heart. 

Although  we  know  that  stages  like  those  just  described 
no  longer  exist  in  living  adult  animals,  it  is  quite  certain  that 
in  these  embryonic  changes  an  early  phylogenetic  history  is 
recapitulated;  that  in  some  past  group  of  animals,  dimly 
foreshadowing  the  vertebrate  type,  a  well-developed  subin- 
testinal vein  existed,  and  that  the  usurpation  of  its  function 
by  the  cardinal  system,  repeated  with  great  faithfulness  to 
detail  in  selachian  embryos,  was  once  actually  experienced 
and  slowly  worked  out  in  adult  animals  through  the  action 
of  natural  selection  or  whatever  other  forces  are  and  have  been 
in  operation  for  the  gradual  improvement  of  organisms. 


342  HISTORY    OF    THE    HUMAN    BODY 

As  has  been  shown  above,  the  Class  of  fishes  comprises 
forms  which  have  remained  at  the  stage  last  described,  tire 
one  in  which  the  cardinal  system  holds  the  supremacy;  but 
by  the  time  the  amphibians  are  reached  there  has  been  another 
usurpation  in  that  partTof  the  body  posterior  to  the  heart, 
and  the  posterior  cardinal  ,systprn  ^S  i"  ^  turn, 
subordinated  to  a  third  svsternj  that  nf  the 


or,  more  briefly,  the  postcava.  How  this  appears  in  full  func- 
tional activity  is  seen  in  the  diagram  representing  the  main 
venous  channels  of  the  urbdele  (Fig.  96,  pc),  where  it  has 
secured  nine-tenths  of  the  traffic  between  the  kidneys  and  the 
heart,  and  allows  but  a  small  part  to  be  conveyed  by  the  pos- 
terior cardinals,  formerly  completely  in  charge  of  this  territory. 
Still  another  rival  of  the  cardinal  system  has  appeared  in  the 
abdominal  vein  (abd),  which  begins  as  two  lateral  veins  that 
issue  from  the  iliacs,  run  along  the  ventral  abdominal  wall 
until  they  meet  in  the  median  line,  and  continue  as  a  single 
vessel  until  opposite  the  liver,  when  the  vessel  leaves  the 
body  wall,  and  enters  this  latter  organ,  forming  a  part  of  the 
hepatic  portal  system. 

The  origin  of  the  vena  cava  historically  cannot  be  now 
learned  from  adult  anatomy,  since  it  undoubtedly  took  place 
in  those  forms  which  successfully  achieved  the  transition  from 
an  aquatic  to  a  terrestrial  life,  or  to,  at  least,  a  paludic  one, 
and,  having  left  for  their  descendants  this  new  world  with 
its  opportunities,  perished  and  left  no  trace  save  in  the  per- 
fected parts  which  render  a  terrestrial  life  possible. 

Here  again,  however,  embryology  furnishes  us  with  some 
information  concerning  at  least  the  place  and  mode  of  origin 
of  this  new  vein,  as  may  be  seen  by  a  comparison  of  the 
diagrams  given  in  Fig  97,  d  and  e,  where  is  shown  the  develop- 
ment of  the  postcava  in  the  lizard.  During  the  early  stages  in 
the  development  of  the  liver  and  its  extensive  system  of  capilla- 
ries, developed  in  association  with  the  portal  system  to  be  con- 
sidered later,  the  postcava  appears  as  a  partially  distinct  element 
in  this  capillary  system,  and  becomes  gradually  more  definite. 
This  vein  grows  posteriorly  and  ultimately  reaches  the  renal 


THE   VASCULAR    SYSTEM 


343 


portal  system  and  the  anastomoses  between  the  caudal  vein 
arid  the  posterior  cardinals.  Here  it  is  united  with  the  pos- 
terior ends  of  these  latter  vessels  and  annexes  them  as  well 
as  the  caudal  vein  to  itself,  thus  establishing  a  single  path 


FIG.  96.  Venous  system  of  urodeles,  based  on  that  of  Desmognathus. 
[In  part  after  (Mrs.)  ANNE  BARROWS  SEEL  YE.] 

ji,  internal  jugular;  je,  external  jugular;  sc,  subclavian;  pc,  postcava;  h,  hepatic; 
pt,  portal;  g,  gastric;  si,  splenic;  abd,  abdominal;  res,  vesical;  ec,  epigastric;  msf 
mesenteric;  il,  iliac;  c,  caudal;  z,  anastomotic  branch  between  the  two  caudals; 
ra,  venae  renaJes  advehentes;  cl,  lateral  cutaneous;  cdp,  cardinalis  posterior;  x, 
anastomotic  branch  between  postcardinal  and  renal.  The  systemic  veins  are  given 
in  black;  the  portal  system  is  in  outline. 


344 


HISTORY    OF    THE    HUMAN    BODY 


from  the  end  of  the  tail,  between  the  kidneys,  to  the  heart. 
The  remainder  of  the  posterior  cardinals,  anterior  to  the  con- 
nection with  the  vena  cava,  becomes  reduced  in  proportion 
to  the  loss  of  function  and  the  two  remain  either  as  small 
but  continuous  vessels,  as  in  the  Amphibia,  or  as  the  azygos 
veins,  which  continue  to  play  a  subordinate  role  by  collecting 


a 


FIG.  97.  Development  of  the  postcava  and  the  hepatic  portal  system 
in  the  lizard  (Lacerta}.  [After  HOCHSTETTER.] 

The  figures  (a)  to  (e)  represent  consecutive  stages  of  development.  s,  sinus 
venosus;  c,  c,  ductus  Cuvieri;  ud,  right  umbilical  vein;  us,  left  umbilical  vein;  omd, 
right  omphalo-mesenteric  vein;  oms,  left  omphalo-mesenteric  vein;  pc,  postcava;  i, 
intestine;  xv  xz,  xv  commissures  between  the  veins  of  the  two  sides. 

the  blood  from  the  sides  of  the  trunk,  especially  from  the 
intrrrostnl  sprvrf^  a  function  which  they  exercise  in  gauropsids 
and  mammals. 

Fig.  98  shows  the  development  of  the  postcava  in  a  mam- 
mal in  which  the  part  played  by  the  posterior  cardinals  is 
especially  emphasized.  In  some  details  the  developmental 


THE   VASCULAR    SYSTEM 


345 


history  in  reptiles,  birds,  and  mammals  differs  a  little;  there 
may,  in  fact,  be  slight  differences  within  the  limits  of  each 
group,  but  the  essentials  are  in  all  cases  as  given  above.  In 
this  specific  case  the  posteriorly  developing  postcava  enters 
the  right  of  two  small  veins  developed  in  the  (here  transitory) 
renal  portal  system  (stage  a).  Stage  b  is  developed  from 
stage  a  through  the  formation  of  a  transverse  anastomosis 
between  this  vein  and  the  two  posterior  cardinals,  with  an  ac- 
companying increase  of  size  in  these  parts.  This  anastomosis, 
x,  divides  the  original  posterior  cardinal  into  two  parts,  y 


FIG.  98.  Development  of  the  postcava  in  mammals.  [After  HOCH- 
STETTER.]  (a)-(d),  Rabbit;1  (e), ' Man. 

j,  anterior  cardinal  (jugular) ;  y,  z,  the  two  £arts  pi^the  posterior  cardinal, 
divided  by  the  anastomotic  vessel  x ;  pc,  postcava;'  zd,  zs,  right  and  left  posterior 
cardinals,  between  the  commissure  x  and  their  union  posteriorly;  in  (e)  the  left 
one  of  these  atrophies  in  part,  the  remainder  becoming  a  portion  of  the  spermatic 
vein  st;  k,  kidney;  s,  suprarenal  body.  .f  ' 

and  2,  of  which  the  former  becomes  reduced  and  forms  the 
azygos,  while  the  latter  develops  as  part  of  the  postcaval  sys- 
tem.  In  stage  -c  the  permanent  kidneys  have  formed,  The 
ureters  from  which  run  through  a  temporary  ring  in  the  part  2, 
a  relation  without  special  significance. 

In  stage  d  an  important  change  is  effected,  first  by  the  fusion 
of  the  two  lateral  elements,  once  the  caiiHal  pnds  ^f  the  pos- 
ter jorjcardinals.  ^and,  second,  by  the  precedence  in  size  and 
function  established  by  the  right  portion  of  the  part  2  anterior 
to  this  fusion.  T<he  kidneys  have  also  moved  anteriorly,  and 
have  developed  the  renal  veins.  The  final -condition  is  shown 


346  HISTORY    OF    THE    HUMAN    BODY 

in  stage  e,  in  which  the  left  limb  of  the  loop  (ss  of  stage  d), 
has  for  the  most  part  disappeared,  while  the  right  has  become 
nearly  median,  and  has  thus  straightened  the  entire  vessel. 
The  left  spermatic  (or  ovarian)  vein,  which  in  stage  d  enters 
the  left  limb  of  the  loop,  has  caused  the  retention  of  that  part 
through  which  its  connection  with  the  main  system  was  origin- 
ally established,  while  the  right  spermatic  vein  enters  the  post- 
cava  directly,  since  this  was  originally  the  part  to  which  it 
was  attached. 

This  last  diagram  (e)  is  taken  from  the  human  embryo, 
since  in  man  the  relation  of  the  iliacs  differs  considerably 
from  that  in  the  rabbit,  from  which  the  other  diagrams  of 
this  series  are  taken.  In  other  respects  there  is  no  appreciable 
difference  between  the  two  forms. 

This  embryological  history  explains  the  composite  structure 
vof  the  postcava  as  seen  in  the  adult.     A^morjy_5__sprout 
"irom  the  liver  capillaries,  it  is  composed  more  posteriorly  of 

vein  connected  with  the  embryonic  kidney  and  a  portion  of 
he  right jDOsterior  cardinal,  and  to  this  is  added,  still  more 
posteriorly,  the  caudal  vein,  primarily  a  portion  of  the  sub- 
ntestinal. 

Concerning  the  abdominal  vein,  which  seems  to  appear  in 
the  amphibians  as  suddenly  as  does  the  postcava,  the  em- 
bryology of  urodeles  shows  it  first  in  the  form  of  paired  lateral 
vessels  lying  in  the  body  wall  and  emptying  into  the  ductus 
Cuvieri  without  connection  with  the  liver.  Its  embryonic 
position  and  relationships  thus  render  it  probable  that  this 
vein  is  the  same  as  the  lateral  vein  of  fishes,  which  likewise 
runs  in  the  body  wall  and  empties  into  the  ductus  Cuvieri 
or  near  it.  The  connections  of  this  vein  with  the  iliacs  pos- 
teriorly and  with  the  liver  anteriorly  appear  later  on  in  em- 
bryonic development  and  are  thus  shown  to  be  secondary 
modifications,  and  not  features  of  the  original  vein. 

Above  the  amphibians  there  is  nothing  which  at  first  sight 
resembles  an  abdominal  vein,  but  the  two  lateral  elements 
of  which  it  is  composed  are  probably  identical  with  the  simi- 
larly related  umbilical  veins,  which  in  the  embryo  supply 


THE   VASCULAR    SYSTEM  347 

the  allantois.  This  membrane  is  itself  the  amphibian  urinary 
bladder  extended  beyond  the  limits  of  the  embryo,  and  there 
is  little  doubt  that  the  two  veins  which  lie  along  its  sides  and 
enter  the  liver,  are  the  primary  lateral  elements  which  in  adult 
amphibians  fuse  to  form  the  median  abdominal  vein.  In 
the  later  history  of  the  umbilical  veins,  the  right  one  becomes 
early  reduced,  and  in  advanced  embryos  the  left  one  alone  re- 
mains; this  collects  all  the  blood  from  the  entire  allantois, 
enters  the  body  at  the  umbilicus,  and  conveys  the  blood  from 
that  point  to  the  liver. 

At  birth,  in  the  case  of  the  mammal,  and  upon  hatching, 
in  reptiles  and  birds,  the  extra-embryonal  portion  of  the  al- 
lantois, together  with  its  blood-vessels,  becomes  pinched  off 
at  the  umbilicus,  but  the  umbilical  vein,  extending  from  the. 
anterior  body  wall  to  liver,  is  retained  as  a  ligament  (lig.  teres 
s.  hepato-umbilicale). 

The  portal  vein,  previously  described,  which  conveys  the 
blood  from  the  intestine  to  the  liver,  is  a  constant  factor 
in  vertebrate  circulation  from  cyclostomes  to  mammals,  and 
as  it  is  essentially  similar  in  all  cases  there  is  but  little  his- 
tory shown  by  the  comparison  of  adult  forms.  The  early 
embryonic  development  shows,  however,  the  method  by  which 
this  portal  system  becomes  established,  and  is  thus  valuable 
in  explaining  the  relation  between  the  original  morphological 
elements  and  the  adult  structures.  The  formation  of  the 
hepatic-portal  system  occurs  always  in  connection  with  the 
two  first  veins  that  appear,  the  vitelline  or  omphalo-mesenteric, 
that  lead  in  from  the  yolk  and  unite  just  posterior  to  they 
heart.  The  intestinal  canal  runs  between  them,  and  from  its 
ventral  aspect  the  liver  buds  out  in  the  form  of  a  connected 
group  of  diverticula,  which  surround  the  veins  in  question  and 
cause  them  to  develop  a  capillary  net-work.  In  fishes  and 
amphibians  the  process  is  a  fairly  simple  one,  but  in  Saurop- 
sida  and  Mammalia  the  matter  becomes  somewhat  more  com- 
plicated by  the  addition  to  tjiis  very  region  of  the  two  um- 
bilical veins,  which  come  in  from  the  allantois.  This  develop- 
ment, in  its  more  complex  form,  is  shown  in  Figs.  97  and  99, 


348  HISTORY    OF    THE    HUMAN    BODY 

which  are  taken  from  the  lizard  and  mammal,  respectively. 
In  a  of  either  figure  are  seen  the  primary  elements  which  enter 
into  the  process,  namely  the  two  omphalo-mesenteric  or  yolk 
veins,  the  two  umbilical  veins,  and  the  alimentary  canal.  The 
ducts  of  Cuvier,  the  cardinal  veins  and  the  sinus  venosus  are 
also  shown,  but  they  are  not  directly  concerned  here. 

The  initiative  is  taken  by  the  two  omphalo-mesenteric  veins 
which  form  successively  three  connecting  bands  that  unite 
them  to  each  other  (xlf  x»,  and  x3  in  Fig.  99).  Of  these, 
x*  is  the  most  posterior,  and  lies  ventral  to  the  intestine;  the 
next,  xz,  is  dorsal ;  and  the  third,  xlf  also  the  most  anterior,  is 
again  ventral.  The  result  of  this  is  the  formation  of  two  rings, 
forming  a  figure  8,  through  which  the  intestine  is  threaded 
f  Fig.  97,  c,  and  Fig.  99,  c).  The  subsequent  suppression  of 
the  left  side  of  the  anterior  ring  and  the  right  side  of  the 
posterior  ring,  as  indicated  in  Fig.  99,  c,  produces  a  single 
large  trunk,  eventually  the  portal,  which  twists  in  a  spiral 
about  the  intestine  (Fig.  97,  d  and  e).  Meanwhile,  the  liver 
has  formed  about  the  two  omphalo-mesenteric  veins  anterior 
to  the  rings,  and  the  necessity  thus  thrust  upon  them  of  sup- 
plying it  with  blood-vessels  results  as  seen  in  Fig.  99,  b  and  c. 
Each  sends  off  lateral  branches  from  the  posterior  side  of  the 
liver  and  gathers  them  up  from  the  anterior  side  until  they 
are  resolved  into  a  mass  of  capillaries,  which  permeate  the 
liver  substance  in  all  directions.  As  a  temporary  necessity, 
to  be  removed  at  the  end  of  embryonic  life,  there  develops 
a  vessel  running  diagonally  through  the  liver  and  extending 
from  the  left  omphalo-mesenteric  vein  posteriorly  to  the  right 
anteriorly,  the  ductus  venosus  Arantii.  Through  this  the 
blood  passes  while  the  liver  tissue  is  still  embryonic,  and  be- 
fore the  capillary  system  within  it  is  fully  established,  but  with 
the  approach  of  birth  the  duct  atrophies  and  the  hepatic  portal 
system  becomes  fully  established. 

The  two  umbilical  veins,  which  appear  in  all  these  figures, 
take  no  part  in  the  formation  of  the  hepatic  system  and  may 
rank  thus  as  structures  wholly  embryonic.  It  is  remarkable, 
however,  that,  as  in  the  case  of  the  omphalo-mesenteric  veins, 


THE   VASCULAR   SYSTEM 


349 


the  two  become  reduced  to  a  single  one,  not  through  so  com- 
plicated a  process  as  in  the  former  case,  but  through  the  com- 
plete suppression  of  the  right  vein.  The  anterior  portion 
of  the  left,  also,  shares  the  same  fate,  and  the  umbilical  vein 


ud         orns  us 
omd 


b 


ud 


2 

\ 
S 

s 
\ 
\ 


us 


ud     ouid         or 

FIG.  99.    Development  of  the  hepatic  portal  system  in  mammals.    [After 

HOCHSTETTER.] 

j,  anterior  cardinal  (jugular);  s,  posterior  cardinal;  c,  ductus  Cuvieri;  omd,  cms, 
right  and  left  omphalo-mesenteric  veins;  ud,  us,  right  and  left  umbilical  veins; 
sz',  sinus  venosus;  xv  xv  xy  commissures  between  the  omphalo-mesenteric  veins  of 
the  two  sides;  y,  y,  and  v,  v,  beginnings  of  the  hepatic  and  portal  capillaries  in 
the  liver;  a,  ductus  venosus  Arantii. 

finally  establishes  a  direct  connection  with  the  heart  through 
the  ductus  venosus  Arantii  (Fig.  99,  c). 

The  fate  of  the  veins  anterior  to  the  heart,  when  compared 
with  that  of  those  posterior  to  it,  is  a  very  simple  one,  for 
while  in  the  latter  region  three  veins  in  succession  have  held 
the  supremacy,  the  sub-intestinal,  the  posterior  cardinal,  and- 
the  postcava,  anteriorly  the  first  to  appear  are  the  anterior 
cardinals,  and  it  is  these  very  vessels  which  in  the  higher  mam- 


350  HISTORY    OF   THE    HUMAN    BODY 

mals,  under  the  name  of  jugulars,  continue  to  serve  in  the 
same  capacity  as  at  first.  The  changes  in  these  parts  are  com- 
paratively slight,  and  consist  mainly  in  the  establishment  of 
two  definite  branches  on  either  side,  the  external  and  internal 
jugulars,  and  in  the  greater  development  of  the  subdavians, 
correlated  with  that  of  the  fore-limbs,  which  gains  for  it  an 
anatomical  rank  equal  to  that  of  the  anterior  cardinal  itself, 
and  suggests  the  name  vena  anonyma  for  that  part  of  the  orig- 
inal anterior  cardinal  below  the  entrance  of  the  subclavian, 
since  it  appears  to  be  formed  by  the  union  of  two  equal  veins. 

These  changes  of  nomenclature,  it  will  be  seen,  are  purely 
anatomical,  and  express  merely  the  apparent  differences  due 
to  change  in  the  caliber  or  the  relative  position  of  the  separate 
portions.  There  are,  however,  a  few  genuine  morphological 
changes  in  the  higher  Classes,  doubtless  rendered  necessary 
by  modifications  in  related  parts.  The  most  striking  of  these 
occurs  in  Man  and  some  other  mammals  and  consists  of  the 
formation  of  a  connection  across  the  middle  line  between  the 
right  and  left  jugulars,  and  the  more  or  less  complete  atrophy 
of  the  right  vena  anonyma.  The  left  innominate  vein  thus 
has  to  convey  all  the  blood  from  both  sides  of  head  and  neck 
and  from  both  anterior  limbs,  and  as  it  is  greatly  increased 
in  size  through  the  assumption  of  this  double  task,  it  early 
received  the  name  of  superior  vena  cava  (anterior  or  precava), 
in  comparison  with  the  inferior  vena  cava  (posterior  or  post- 
cava),  which  enters  the  right  atrium  near  it. 

A  simple  modification  takes  place  in  the  mammalian  pos- 
terior cardinals.  The  formation  of  a  transverse  connection 
between  the  two  allows  one  of  them  to  assume  the  function 
of  conveying  to  the  heart  the  blood  collected  by  the  metameric 
branches,  and  permits  the  other  to  sever  its  anterior  connection 
with  the  heart.  Although  there  is  much  variation  in  this, 
the  more  common  condition  in  Man  consists  of  the  preserva- 
tion of  the  posterior  cardinal  on  the  right  side  in  its  entirety, 
into  which  the  other  empties  by  means  of  the  transverse  con- 
necting vein,  and  is  deprived  of  all  direct  connection  with  the 
heart.  The  first,  or  complete  vein,  was  called  the  azygos, 


THE   VASCULAR    SYSTEM 


or  unpaired,  vein,  from  a 
mistaken  early  notion  that 
it  had  no  mate  on  the 
other  side;  the  other,  the 
incomplete  one,  was  named 
the  hemiazygos.  What  lit- 
tle applicability  these  names 
may  possess,  however,  is 
confined  to  Man  and  allied 
forms,  since  in  many  other 
mammals  quite  different 
results  obtain.  Thus,  in 
rabbits,  the  main  trunk  of 
the  hemiazygos  entirely 
disappears,  and  the  azygos 
receives  the  intercostal 
veins  from  both  sides, 
while  in  the  pig  the  re- 
verse is  the  case  and  it  is 
the  hemiazygos  which  per- 
sists. These  relations  are 
extremely  variable,  even 
in  Man,  where  the  oc- 
casional conditions  classed 
as  anomalies  receive  their 
complete  explanation 
through  the  morphologi- 
cal history  of  the  region. 
As  a  review  of  the  ven- 
ous system  of  Man,  with 
the  morphological  signifi- 
cance of  the  principal 
parts,  there  may  here  be 
presented  Fig.  100,  which 
shows  the  course  of  the 
main  trunks  in  the  adult, 
the  older  parts  which  have 


ILint 


FIG.  100.  Diagram  showing  the 
history  of  the  venous  system  in  Man. 
[Modified  from  THANE,  in  QUAIN'S 
Anatomy.] 

Primitive  vessels  that  become  atrophied 
are  marked  by  cross  lines;  those  secon- 
darily established  are  marked  by  rows 
of  dots. 


352  HISTORY    OF   THE    HUMAN    BODY 

atrophied  and  the  new  connections  which  have  been  added. 
In  the  background  may  be  seen  the  primitive  cardinal 
system  of  fishes,  and  perhaps  in  the  minute  caudal  vein,  even 
a  trace  of  the  still  earlier  sub-intestinal  system.  Here,  as 
elsewhere,  we  receive  the  distinct  impression  of  the  constant 
modification  of  old  relations  to  fit  new  conditions,  and  we 
see  the  numerous  mechanical  difficulties  which  are  the  in- 
evitable result  of  such  a  process.  Here  and  there,  where 
a  difficulty  is  sufficiently  great  to  interfere  seriously  with  the 
preservation  of  the  race,  it  is  overcome,  if  possible ;  if  not, 
the  race  dies  out;  but  generally  the  adaptation  is  fairly  com- 
plete, and,  while  we  may  never  know  of  the  countless  forms 
which  were  lost  in  the  sifting  process,  those  that  survive 
are  not  seriously  handicapped  by  the  circuitous  paths  through 
which  their  organs  have  arrived  at  their  final  condition,  and 
the  atrophied  rudiments  form  no  serious  disadvantage  to 
the  organism. 

As  one  example  of  the  slowness  of  the  adaptation  where 
the  disadvantage  is  inconsiderable,  we  have  the  case  of  the 
vena  anonyma  of  the  left  side.  Although  the  plan  by  which 
all  the  venous  blood  is  received  upon  the  right  side  of  the 
heart  is  inaugurated  by 'the  amphibians,  there  is,  in  the  an- 
terior cardinal  system,  no  anatomical  recognition  o-f  this 
throughout  amphibians,  reptiles,  and  birds,  all  of  which  still 
possess  symmetrical  vena  anonymce,  and  the  left  one  is  forced 
to  bring  its  blood  over  to  the  right  side.  First  among  the 
mammals  comes  the  formation  of  an  obliquely  placed  trans- 
verse connective,  which  allows  the  establishment  of  a  true  vena 
cava  anterior,  and  rectifies  the  slight  mechanical  disadvantage. 
The  mere  fact  that  this  condition  continues  so  long  without 
readjustment  suggests  that  the  disadvantage  must  be  ex- 
ceedingly slight,  far  too  inconsequent  to  come  under  the  direct 
control  of  Natural  Selection;  and  the  bettered  condition  in 
these  mammals  cannot  fail  to  suggest  the  result  of  mechanical 
causes,  operating  continually  for  a  long  time,  and  always  in 
the  same  direction. 

The  heart  is  in  origin  nothing  but  a  localized  portion  of 
a  large  blood-vessel,  the  walls  of  which  develop  a  thickened 


THE    VASCULAR    SYSTEM 


353 


muscular  layer,  transforming  it  into  a  pulsating  engine  to 
promote  the  flow  of  blood.  Similar  pulsating  vessels  occur  in 
all  animals  furnished  with  a  closed  circulation,  usually  a  single 
one,  but  in  some  cases  several  in  number.  Thus,  in  arthropods 
and  molluscs  there  is  a  single  median  heart,  located  dorsally 
upon  the  main  blood  channel  in  that  region,  but  in  annelids 
several  of  the  lateral  commissures  are  enlarged  and  function 
as  hearts.  In  vertebrates  the  hypertrophied  region  which 

forms  the  heart  is  located 
along  the  median  ventral 
blood-vessel  formed  by  the 
union  of  the  two  vitelline 
veins,  and  involves  its  pos- 
terior portion.  This  brings 
it  topographically  very  far 
anterior,  just  back  of  the 
gill  region,  and  this  primary 
position  is,  indeed,  that  per- 
manently retained  in  fishes 
and  amphibians;  but  in  rep- 
tiles, birds,  and  mammals  it 
suffers  a  considerable  change 
of  location  in  a  posterior  di- 
rection and  comes  to  lie  in 
a  thoracic  cavity,  formed  by 
the  ribs  and  sternum,  with 
some  participation  of  parts  of  the  shoulder-girdle. 

In  its  first  stage,  as  shown  by  Amphioxus  and  in  early  em- 
bryos, the  heart  is  still  a  straight  tube,  formed  posteriorly 
by  the  joining  of  the  hepatic  veins,  and,  in  true  vertebrates. 
the  two  ducts  of  Cuvier  and  the  two  vitelline  veins  also,  the 
last  being  embryonic  and  transitory.  The  chamber  into  which 
these  vessels  empty  soon  differentiates  off  from  the  rest  as 
the  sinus  venosus,  and,  in  like  manner,  by  the  formation  of 
transversely  placed  constrictions,  there  are  added  successively 
an  atrium,  a  ventricle,  and  a  conus  arteriosus,  the  latter  being 
continued  into  the  median  artery  that  supplies  the  gills. 

The  next  advance  is  seen  in  a  flexion  of  the  axis  of  the 


FIG.   101.     Four  stages  m  the  de- 
velopment of  the  amniote  heart. 


354  HISTORY    OF    THE    HUMAN    BODY 

heart  into  the  form  of  an  S-shaped  tube,  the  bending  being 
in  such  a  way  that  the  sinus  venosus  is  dorsal  and  the  conns 
arteriosus  ventral,  the  atrium  anterior  and  the  ventricle  pos- 
terior, a  stage  represented  in  adult  fishes  and  in  the  embryos 
of  higher  forms.  The  atrium  increases  in  width  more  than  the 
other  parts  and  forms  two  lateral  recesses  or  broad  diverticula, 
which,  from  the  ventral  aspect,  appear  on  either  side  of  the 
conus  arteriosus,  suggesting  the  division  into  two  separate 
compartments,  which  is,  in  point  of  fact,  the  next  advance. 
Furthermore,  the  several  parts  brought  into  contact  by  the 
flexion  become  permanently  adherent  to  one  another  and  the 
heart  becomes  molded  into  a  more  compact  organ. 

In  the  tailed  amphibians  a  new  physiological  moment  is 
introduced  by  the  reception  for  the  first  time  of  arterial  blood, 
which  comes  from  the  skin  and  lungs  through  the  great  pul- 
mo-cutaneous  veins  and  enters  the  left  side  of  the  atrium 
(Fig.  102,  B).  The  anatomical  response  to  this  consists  of 
the  division  of  the  atrium  into  right  and  left  portions,  the 
former  for  the  reception  of  impure,  and  the  latter  for  pure 
blood.*  From  now  on  the  sinus  venosus  plays  a  subordinate 
role  and  consists  merely  of  a  vestibule  of  entrance  for  the 
systemic  veins,  applied  to  the  dorsal  side  of  the  right  atrium. 
In  birds  and  mammals  it  is  no  longer  distinct.  The  ventricle 
is  still  undivided  in  the  urodeles,  but  in  the  tailless  forms, 
probably  as  a  further  response  to  the  new  physiological  con- 
dition, a  partial  septum  appears  in  this,  which  suggests  a  di- 
vision into  right  and  left  ventricles,  but  contains  a  large 
opening  through  which  the  two  kinds  of  blood  still  mingle 
(Fig.  102,  C).  In  these  animals  also,  the  complete  differen- 
tiation of  the  third  and  .fourth  arterial  arches'  as  aorta  and 
pulmonary  artery,  respectively,  leads  to  a  longitudinal  division 
of  the  conus  arteriosus  by  means  of  two  longitudinal  folds 
placed  opposite  to  one  another  which  grow  from  the  inner 

*  In  the  urodeles  the  septum  atriorum  is  not  complete,  but  possesses  a 
few  secondary  perforations.  In  the  lungless  salamanders  the  left  atrium, 
into  which  the  pulmonary  veins  would  empty  under  other  circumstances,  is 
suppressed. 


THE   VASCULAR    SYSTEM 


355 


walls,  meet,  and  fuse.  This  causes  a  complete  separa- 
tion of  these  main  vessels  as  far  back  as  the  ventricle,  and 
their  exits  from  this  chamber  are  placed  in  such  a  way  that 
the  blood  from  the  right  side,  which  is  mainly  imgure,  passes 


FIG.  102.     Diagrams  of  the  heart  and  its  compartments  in  the  different 
vertebrate  Classes.     [In  part  after  WIEDERSHEIM.] 

(A)    Fish;    (B)    Urodele;    (C)   Anuran;    (D)    Reptile;    (E)    Bird;    (F)    Mammal. 


356  HISTORY   OF   THE    HUMAN    BODY 

into  the  pulmonary  artery,  a  condition  which  continues  as  a 
permanent  one  from  this  point  on.  The  relation  of  the  two 
aortic  arches,  however,  is  not  so  perfect,  for  they  cross  in 
such  a  way  that  while  the  right  one  contains  mainly  pure 
blood,  the  left  one,  in  company  with  the  pulmonary  artery, 
collects,  in  part,  impure  blood  from  the  right  side.  This 
causes  a  mixture  of  blood  in  this  arch  as  well  as  in  the  main 
aorta,  and  the  result  is  that  the  blood  is  never  wholly  aerated, 
a  condition  which  does  not  allow  the  establishment  of  a  con- 
stant bodily  temperature,  but  compels  the  animal  to  be  "  cold- 
blooded," or  more  correctly,  poikilothermous,  that  is,  change- 
able in  temperature  in  more  or  less  accordance  with  its  sur- 
roundings. 

The  heart  of  reptiles  (Fig.  102,  D)  is  similar  to  the  last 
save  that  the  ventricular  partition  is  more  extensive,  though 
still  incomplete,  therefore  the  relation  of  these  animals  to  ex- 
ternal temperature  is  the  same  as  in  amphibians;  in  birds 
(Fig.  1 02,  E),  however,  the  opening  between  the  ventricles 
closes,  thus,  for  the  first  time,  completely  separating  the  two 
kinds  of  blood.  Probably  correlated  with  this  is  the  complete 
atrophy  of  the  left  aortic  arch,  leaving  the  right  one  as  the 
only  connection  between  heart  and  median  aorta. 

Mammals  (Fig.  102,  F),  although  but  indirectly  related  to 
the  birds,  have  accomplished  the  same  separation  of  pure  and 
impure  blood,  corresponding  to  the  left  and  right  halves  of  the 
heart,  respectively,  a  relation  which  is  still  further  emphasized 
by  the  main  blood-vessek,  the  sy^tejnjc__v^nous^  trunks  as- 
suming a  position  on  the  Ight,  in  connection  with  the  atrium 
of  that  side,  and  the  mairmaorta  being  on  the  left,  instead  of 
the  right.  In  both  birds  arid  mammals,  then,  the  tissues  are 
supplied  with  pure  blood  and  are  thus  enabled  to  maintain  a 
constant  body  temperature,  which  fluctuates  butfa  few  degrees, 
and  is  usually  higher  than  the  external  temperature.  There 
thus  comes  to  be  developed  in  the  two  most  highly  specialized 
Classes  of  vertebrates,  birds  and  mammals,  a  complete  and 
almost  symmetrical  double  heart,  of  which  the  right  half  is 
associated  with  the  venous,  the  left  with  the  arterial,  blood. 


THE    VASCULAR    SYSTEM  357 

In  both  the  symmetry  is  not  a  primary,  but  a  secondary  one; 
the  heart  begins  and  ends  with  four  chambers,  it  is  true,  but 
they  do  not  at  all  correspond,  the  latter  form  resulting  from 
the  suppression  of  two  of  the  first  and  the  subdivision  of  the 
remaining  two*.  <- 

The  lymphatic  function,  that  which  cares  for  the  blood 
components  which  become  infiltrated  into  the  tissues,  and  re- 
turns them  to  the  general  circulation,  is  performed,  not  only 
by  spaces  and  vessels  primarily  formed  for  that  purpose,  but, 
in  the  lower  forms  at  least,  by  the  most  of  the  spaces  and 
lacunae  of  the  body  that  form  parts  of  other  systems.  In 
fishes  and  amphibians  a  system  of  lymph  channels  becomes 
developed  in  the  loose  connective  tissue  that  forms  an  external 
sheath  for  the  larger  blood-vessels,  and  here  the  lymph,  not 
confined  within  special  walls,  is  intercellular  and  circulates 
freely  within  the  meshes  formed  by  the  branching  connective- 
tissue  cells.  One  of  the  largest  of  these  lymph  channels  is  the 
subvertebral  ^space,  which  enfolds  the  aorta.  The  lympatics 
that  collect  the  digested  food  (chyle)  from  the4  intestine  com- 
municate, either  directly  or  indirectly,  with  this  channel,  which 
thus  forms  a  very  primitive  thoracic  duct. 

A  second  series  of  lympliatic^spaces  Ts" found  immediately 
beneath  the  membranes  -lining  the  great  serous  cavities  of  the 
body;  such  are  the  sub-peritoneal  spaces  in  the  walls  of  the 
coelom,  the  sub-dural  and  inter-dural  spaces  connected  with 
the  membranes  that  invest  the  central  nervous  system,  and  the 
peri-  and  endo-lymphatic  spaces  of  the  inner  ear.  Still  others 

*  During  fetal  life  there  is,  in  the  mammalian  heart,  an  opening  in  the 
interatrial  septum,  through  which  the  blood  of  the  two  atria  freely  mixes. 
This  is,  however,  a  secondary  condition,  developed  in  adaptation  to  the 
fetal  circulation,  as  is  shown  by  the  fact  that  the  septum  is  first  com- 
pleted before  the  opening  is  formed.  In  the  embryos  of  the  monotremes 
and  marsupials  there  are  several  small  foramina  instead  of  one  big  one; 
which  is  similar  to  the  condition  found  in  urodeles,  where  the  perforations 
are  also  secondary.  The  large  foramen  in  the  mammals,  the  foramen 
ovale,  persists  normally  in  the  human  infant  for  a  few  days  after  birth; 
but  is  occasionally  pemianent,  producing  the  condition  known  as  cyanosis, 
in  which  the  individual  suffers  continually  from  the  presence  of  venous 
blood  in  the  arterial  system. 


358  HISTORY    OF    THE    HUMAN    BODY 

are  intermuscular  or  sub-fascial,  their  locations  being  desig- 
nated by  their  names,  and  in  tailless  amphibians  there  is  found 
an  extensive  series  of  sub-cutaneous  lymph-sacs,  some  of  great 
extent. 

These  various  spaces  are  in  communication  with  one  another 
and  usually  communicate  with  the  venous  system  in  four 
places:  anteriorly  with  the  two  jugulars  at  or  near  their  union 
with  the  subclavians;  and  posteriorly  with  either  the  caudal 
vein  or  the  posterior  cardinals  near  the  entrance  of  the  iliacs. 
It  is  to  be  noted  that  these  four  points  at  which  the  lymphatic 
and  circulatory  systems  communicate  are  associated  with  the 
four  limbs,  and  although  the  number  of  these  points  of  com- 
munication is  decreased  in  the  higher  vertebrates  through  a 
suppression  in  the  adult  of  certain  of  these,  there  are  no  new 
ones  formed,  and  the  lymphatic  system,  even  in  its  most  spe- 
cialized form,  still  follows  in  this  particular  the  lines  laid  down 
for  it  from  the  first. 

In  fishes  the  lymphatic  vessels,  near  their  entrance  into  the 
veins,  enlarge  into  thin-walled  sinuses,  organs  which  in  tail- 
less amphibians  develop  into  pulsating  sacs  with  muscular 
walls,  the  so-called  lymph-hearts,  the  action  of  which  furthers 
the  flow  of  the  lymph  (Fig.  163).  Each  lymph -heart  pos- 
sesses a  single  venous  ostium,  by  which  the  sac  communicates 
^wlth  the  vein,~and  several  lymphatic  ostiq,  through  which  the 
sac  receives  the  fluid  from  as  many  lymphatic  vessels.  The 
forrneY^)peningjs^  equipped  with  two  semi-lunar  valves  to  pre- 
vent filling  the  sac  with  blood  during  its  expansion,  but  the 
lymphatic  ostia  are  without  special  valves.  The  tailed  am- 
phibians seem  to  lack  the  anterior  pair  of  lymph-hearts,  but 
here,  in  addition  to  the  posterior  pair,  a  series  of  small  pul- 
sating sacs  occur  along  the  lateral  line. 

Progress  in  the  history  of  the  lymphatic  system  among  the 
higher  vertebrates  is  shown  along  two  directions :  first,  in  the 
formation  of  more  and  more  vessels  with  walls  of  their  own, 
the  definite  lymphatics,  and,  second,  by  the  reduction  of  the 
lyrriph-hearts.  Thus  in"  reptiles  only  the  posterior  lymph- 
hearts  persist  in  the  adult,  and  the  same  appear  in  birds  dur- 


THE   VASCULAR    SYSTEM 


359 


fe. 


FIG.  103.  Venous  system  of  frog,  to  show  the  two  pairs  of  pulsatile 
lymph-hearts;  an  anterior  pair  opening  into  the  vertebral  veins,  and  a 
posterior  pair  opening  into  a  cross  vein  between  the  ischiadic  and  femoral 
veins. 

ji,  internal  jugular;  je,  external  jugular;  ubsc,  subscapular;  sc,  subclavian; 
cu,  cutaneous;  pul,  pulmonary;  h.  hepatic;  card,  cardiac;  po,  hepatic  portal; 
abd,  abdominal;  r.  adv,  afferent  reoal;  is,  ischiadic;  fe,  femoral. 


360  HISTORY   OF    THE    HUMAN    BODY 

ing  development,  but  are  here  transitory  structures  and  do 
not  survive  embryonic  life.  The  division  of  the  subvertebral 
space  into  two  lateral  thoracic  ducts  is  inaugurated  in  croco- 
diles and  turtles,  and  becomes 'definite  in  birds;  in  these  the 
chyle  from  the  intestines  is  collected  and  emptied  into  the 
veins  at  the  junctu7e~~6:T~jugular  and  subclavian.  There  are 
also  in  all  Sauropsida  posterior  connections  with  the  venous 
system,  but  only  in  reptiles  do  pulsating  hearts  persist  at  these 
points. 

Th£  above  phylogenetic  history  of  the  lymphatic  system  is 
well  recapitulated  during  the  embryonic  development  of  mam- 
mals, and  the  adult  condition  is  best  understood  by  tracing 
the  steps  in  this  development  (Fig.  104).  As  in  the  case  of 
the  circulatory  system,  the  preservation  of  so  many  phyloge- 
netic steps  in  this  developmental  history  is  doubtless  due  to 
the  continual  functional  activity  of  this  system  from  an  early 
embryonic  period,  its  usefulness  at  all  stages  preventing  the 
customary  degeneration  of  transitory  structures.*  The  lym- 
phatic system  first  appears  in  the  form  of  a  pair  of  tiny  diver- 
ticula  which  bud  out  from  the  venous  system  at  the  angle 
formed  by  the  meeting  of  the  jugular  and  subclavian  veins. 
From  these  develops  an  anterior  pair  of  lymph-hearts,  in  lo- 
cation similar  to  those  of  lower  forms,  but  without  muscular 
walls;  and  from  these  chambers  as  centers,  definite  lymphatic 
vessels  begin  to  develop,  growing  from  their  free  ends  and 
gradually  invading  the  surrounding  tissues.  At  a  slightly 
later  period  a  pair  of  posterior  hearts  appears,  and  from  these 
in  the  same  way  there  grow  out  branching  lymphatics. 

It  is  at  this  period  that  the  thoracic  ducts  make  their  ap- 
pearance, starting  from  the  vessels  connecting  the  lymph- 
hearts  with  the  veins  and  growing  posteriorly,  following  the 
aorta.  There  are  two  of  these,  which  at  first  are  about  equal 
in  size,  but  soon  the  left  one  gains  the  superiority,  and  branches 
to  supply  each  side,  while  the  right  one  remains  small.  The 

*  An  embryo  in  which  the  lymphatic  system  is  not  in  full  functional 
activity  becomes  cedematous,  and  in  extreme  cases  the  result  is  a  spheri- 
cal ball,  without  indication  of  the  normal  shape. 


THE    VASCULAR   SYSTEM 


362  HISTORY   OF   THE    HUMAN    BODY 

left  duct  thus  comes  to  furnish  two  long  lateral  vessels,  one 
on  each  side  of  the  aorta,  which  extend  posteriorly  until  they 
enter  the  posterior  system  near  the  lymph-hearts,  thus  uniting 
the  two  systems ;  and  the  severance  of  the  original  connection 
between  the  posterior  lymph-hearts  and  the  posterior  cardinals, 
now  the  postcava,  renders  the  posterior  system  dependent  upon 
the  anterior  connections.  During  later  development  several 
important  changes  take  place.  The  lymphatics  gradually 
spread  from  the  four  primary  centers  over  the  entire  body, 
the  original  anterior  and  posterior  systems  communicating 
at  every  point  until  all  distinction  between  the  two  becomes 
lost;  the  lymph-hearts  lose  their  identity;  two  posterior  en- 
largements, the  cisterns  [receptacula]  chyli,  appear  in  the 
posterior  parts  of  the  lateral  thoracic  ducts ;  and  the  growing 
intestine  becomes  supplied  with  lymphatics  from  the  left  lateral 
thoracic  duct.  During  the  growth  of  the  lymphatic  vessels 
numerous  secondary  centers  are  formed,  from  which  several 
vessels  radiate  in  various  directions,,  and  about  these,  through 
participation  of  the  connective  tissue  and  the  blood-vessels, 
there  develop  the  characteristic  lymphatic  nodes  or  "  glands." 
Similar  centers,  developed  in  the  course  of  the  left  lymphatic 
duct  as  it  branches  within  the  mesentery,  form  the  mesenteric 
glands.  In  these  glands  the  physiological  unit  seems  to  be 
a  tuft  of  blood  capillaries  surrounded  by  lymphatic  vessels, 
the  whole  packed  in  a  loose  connective  tissue,  the  lymphoid  or 
adenoid  tissue.  Such  a  structure  is  called  a  lymph  follicle,  and 
a  node  may  consist  of  a  large  number  of  such  structures. 
Within  the  interstices  of  the  lymphoid  tissue  occur  large  quan- 
tities of  lymphocytes,  or  wandering  cells  that  appear  to  be 
identical  with  the  leucocytes  or  white  blood  corpuscles,  and 
although  proof  is  thus  far  wanting,  it  is  probable  that  the 

^  lymphatic  nodes  form  one  of  the  localities  in  which  these  cells 
are  formed. 

In  the  walls  of  the  colon  of  mammals  appear  aggregations 
of  nodules,  similar  to  those  connected  with  the  lymphatic 
system,  and  forming  large  areas  known  as  noduli  lymphatici 

y  dggvcgati  \Peyer' s  patches'].    Many  attempts  have  been  made   I 


THE   VASCULAR   SYSTEM  363 

to  connect  these  with  the  lymphatic  nodes,  the  extreme  theory 
being  that  here  is  the  center  of  origin  for  all  the  latter, 
and  that  they  migrate  from  these  patches  during  development, 
first  invading  the  mesentery  and  forming  the  mesenteric 
glands;  thence  passing  from  these  along  the  lymphatic  chan- 
nels to  all  parts  of  the  body.  As  the  nodules  of  Peyer's 
patches  are  endodermic  in  origin,  it  would  follow  that,  with 
such  an  origin,  all  of  the  lymphatic  nodes,  wherever  found, 
must  be  also  endodermic,  and  it  was  thus  held  that  we  had  here 
an  example  of  elements,  originally  endodermic,  wandering 
over  the  entire  body  and  invading  practically  all  the  tissues. 
The  more  recent  exposition  of  the  development  of  the  lym- 
phatic system,  as  given  above,  renders  such  theories  no  longer 
tenable  and  shows  the  lymphatic  system,  at  least  in  mammals, 
to  be  a  definite  system,  budding  out  from  that  of  the  circula- 
tion, and,  like  it,  mesenchymatous  in  origin.  Whether  this 
is  the  case  in  the  lower  Classes  of  vertebrates  and  whether 
the  various  spaces  utilized  by  the  lymph  can  be  thus  derived 
cannot  be  ascertained  until  the  development  of  the  lymphatics 
in  these  forms  is  as  well  known  as  it  is  in  mammals ;  but  with 
our  present  knowledge  it  seems  probable  that  certain  definite 
channels  that  possess  walls  of  their  own,  like  the  sub-verte- 
bral space,  are  produced  as  outgrowths  from  the  blood-vessels, 
and  that  these  enter  into  secondary  communication  with  nu- 
merous irregular  spaces  which  can  well  be  utilized  as  adjuncts 
of  the  lymphatic  system  until  their  function  can  be  supplied 
by  definite  lymphatic  vessels. 

Aside  from  the  solitary  and  aggregated  nodules,  both  of 
which  appear  to  be  centers  of  origin  of  leucocytes,  there  are 
numerous  other  places  in  which  the  cellular  constituents  of 
the  blood  are  developed.  Many  of  these,  as  in  the  case  of  the 
aggregated  nodules  of  the  intestine,  are  developed  within  the 
wall  of  the  alimentary  canal  and  are  therefore  endodermic  in 
origin.  These  include  the  tonsils,  the  thymus,  and  thyreoid 
glands,  the  associated  epithelial  bodies,  and  perhaps,  the  spleen. 
The  marrow  of  the  bones  is  especially  important  in  this  re- 
spect, and  develops  large  quantities  of  the  blood-cells,  espe- 


364  HISTORY   OF   THE    HUMAN    BODY 

dally  erythrocytes  (red  blood  corpuscles).  Although  lym- 
phatic vessels  secondarily  invade  these  organs,  and  are  hence 
found  in  the  adult  in  close  association  with  them,  they  are 
not  to  be  considered  parts  of  the  lymphatic  system  any  more 
than  one  would  consider  the  kidneys  a  part  of  the  circulatory 
system  because  their  tissues  are  invaded  by  special  forms  of 
blood-vessels. 

In  their  function  as  formative  nidi  for  the  cellular  ele- 
ments of  the  blood  and  lymph  these  organs  form  physiologi- 
cally important  auxiliaries  to  the  vascular  system  as  a  whole, 
but  belong  elsewhere  in  their  anatomical  and  developmental 
affinities. 


CHAPTER   IX 
THE  URO-GENITAL  SYSTEM 

"  We  do  not  draw  conclusions  with  our  eyes,  but  with 
our  reasoning  powers,  and  if  the  whole  of  the  rest 
of  living  nature  proclaims  with  one  accord  from  all 
sides  the  evolution  of  the  world  of  organisms,  we  can- 
not assume  that  the  process  stopped  short  of  Man. 
But  it  follows  also  that  the  factors  which  brought 
about  the  development  of  Man  from  his  Simian  an- 
cestry must  be  the  same  as  those  which  have  brought 
about  the  whole  of  evolution." 

AUGUST  WEISMANN,  The  Evolution  Theory, 
Authorized  translation.     Vol.  II,  p.  393. 

THE  two  systems  included  under  this  compound  name  are 
those  concerned  with  the  very  diverse  functions  of  the  elimi- 
nation of  liquid  waste  and  the  formation  of  new  individuals. 
They  are,  however,  closely  associated  topographically  "and 
usually  possess  certain  parts  in  common,  so  that  they  belong 
together  anatomically,  though  not  physiologically.  They  pos- 
sess also  important  relations  to  the  body  cavity  or  ccelom 
(more  strictly,  metaccele),  and  as  the  latter  is  in  by  no  means 
a  primitive  condition  in  either  Amphioxus  or  the  cyclostomes, 
recourse  must  be  had  to  early  embryonic  stages  and  also  to 
invertebrates  in  order  to  reconstruct  the  early  period  of  the 
history  of  these  organs,  a  knowledge  necessary  for  the  ex- 
planation of  many  of  the  existing  relationships.  To  begin 
with,  let  us  suppose  an  animal  built  on  the  plan  of  a  gastrula, 
but  with  a  space  left  between  the  endoderm  and  the  ectoderm 
(Fig.  105,  A;  also  Plate  I).  Such  an  animal  consists  of  two 
tubes,  one  inside  the  other,  and  two  cavities.  The  two  tubes 
are,  of  course,  alimentary  canal  and  body  wall,  and  the  two 
cavities  are  the  digestive  cavity  (gastrocwle)  and  the  primary 
body  cavity  (protoccele) .  Within  this  protoccele  are  contained 
two  sets  of  organs,  each  opening  to  the  exterior,  excretory  tu- 

365 


366 


HISTORY    OF    THE    HUMAN    BODY 


bules  (nephridia)  and  germ  glands  (gonads).  (Fig.  105, 
B.)  If  the  animal  is  unsegmented,  i.  e.,  consists  of  a  single 
segment,  there  is  a  single  pair  of  each ;  if  it  is  multisegmented, 
there  is  a  pair  of  each  for  each  segment. 

Each  nephridium  consists  of  a  free  tubule  whose  function 
is  to  extract  from  the  protoccele  certain  waste  products  in 
liquid  form,  a  function  which  it  performs  in  part  by  a  ciliated 
funnel-shaped  opening,  the  nephrostome,  and  in  part  by  the 
physiological  action  of  the  cells  of  which  its  walls  are  com- 
posed. In  its  simplest  form  it  is  straight  or  slightly  curved, 
but  it  is  more  usually  coiled  in  order  to  increase  its  length, 


B 


FIG.  105.  Diagrams  to  illustrate  the  prevertebrate  history  of  the  nephri- 
dia, the  gonads  and  the  ccelom. 

(A)  Stage  of  gastrula,  with  two  germ  layers,  a  gastrocoele  (g),  and  a  primary 
body  cavity  (/>).  (B)  Here  a  third  layer  has  appeared  in  the  form  of  paired 
gonadic  sacs  (m),  and  paired  nephridial  tubules  (f) ;  with  external  openings  at 
o  and  x  respectively.  The  nephridia  are  furnished  with  an  inner  opening,  the 
nephrostom  (n).  (C)  In  this  the  gonadic  sacs  (m)  have  expanded  and  form  the 
definite  ccelom,  limiting  the  primary  body  cavity  to  a  series  of  small  spaces  in  all 
parts  of  the  body.  The  nephridia  open  internally  into  these  sacs,  and  their  outer 
ends  open  into  a  longitudinal  duct  (#). 

and  hence  its  functional  efficiency  within  the  prescribed  limits. 
Nephridia  of  this  type  are  frequent  among  invertebrates. 

The  other  sort  of  organ,  the  gonad,  has  the  form  of  a 
simple  epithelial  sac,  with  a  narrow  duct.  Its  walls  are  con- 
stantly proliferating  and  furnish  cells  which  project  into  the 
interior  and  finally  become  free,  passing  out  through  the 
duct.  These  are  the  germ-cells,  and  may  be  either  ova  or 
spermatozoa,  the  product  respectively  of  female  and  male 
parent  individuals.  Gonads  of  this  character  are  frequently 
found  among  invertebrates,  often  in  quite  typical  form. 


THE   URO-GENITAL    SYSTEM  367 

Taking  now  an  animal  of  the  type  shown  in  Fig.  105,  B, 
with  a  continuous  protocoele  and  with  a  metamerism  marked 
by  several  successive  pairs  of  associated  nephridia  and  gonads, 
imagine  the  result  of  a  gradual  and  equal  expansion  of  the 
latter  until  they  attain  the  furthest  possible  limits  (Fig.  105, 
C;  also  Plate  II).  The  protoccele  becomes  suppressed  and  in 
its  place  exists  a  series  of  paired  chambers,  metacoeles,  each 
pair  in  contact  with  the  previous  and  succeeding  ones  and  en- 
closing between  them  the  alimentary  canal.  This  latter  part  is 
thus  hung  between  dorsal  and  ventral  partitions,  the  mesenter- 
ies, each  double  and  composed  of  the  walls  of  the  gonadic  sacs ; 
also  each  pair  of  cavities  is  separated  from  the  next  by  similar 
double  partitions  which  form  intersegmental  diaphragms  or 
dissepiments.  Each  lateral  metacoele  opens  to  the  exterior 
by  the  opening  which  was  once  that  of  the  gonadic  duct.  Thus 
far  no  provision  has  been  made  for  the  nephridia,  which,  with 
the  suppression  of  the  protocoele,  find  themselves  deprived  of 
their  original  function.  Their  fate  is,  however,  simple  and 
obvious,  for  they  receive  an  investment  of  the  gonadic  wall, 
and  although  lying  between  this  and  the  outer  body  wall, 
project  into  the  metacoelic  cavity,  with  which  they  communi- 
cate through  the  ciliated  nephrostome  at  their  free  end.  But- 
one  further  modification  is  necessary  to  adjust  matters  to 
the  new  conditions,  and  that  concerns  the  walls  of  the  meta- 
cceles.  When  in  their  original  condition  as  the  walls  of 
small  sacs  employed  for  the  production  of  germ  cells,  every 
portion  of  their  surface  is  needed  for  the  production  of  these 
latter  elements,  but  when  expanded  to  their  final  dimensions, 
they  become  mesenteries,  dissepiments  and  the  lining  mem- 
brane of  body  walls,  and  form  a  thin  and  firm  membrane,  the 
peritoneum,  while  their  original  reproductive  function  is  con- 
fined to  certain  restricted  areas,  situated  near  the  dorsal  me- 
senteries. These,  by  a  slight  evagination,  produce  rounded 
elevations  that  project  into  the  lumen  of  the  cavity  and  form 
specialized  germ  glands,  the  ovaries  and  the  testes.  Their  prod- 
ucts, when  mature,  separate  from  their  place  of  origin  and 
wander  freely  about  in  the  metaccelic  cavity.  From  this  they 


368  HISTORY    OF    THE    HUMAN    BODY 

have  two  avenues  of  escape,  the  nephridia  and  the  original 
openings  of  the  gonadic  sacs,  and  while  so  far  as  is  known 
no  animal  exists  that  utilizes  both  methods,  either  one  may 
become  specialized  to  subserve  this  function.  Furthermore, 
in  an  animal  with  many  somites  it  is  not  necessary  that  ovaries 
or  testes  should  develop  in  each  pair  of  metaccelic  sacs,  but 
these  may  be  confined  to  a  few  pairs  or  even  a  single  pair, 
in  which  either  the  nephridia  or  the  gonadic  openings  develop 
into  special  excurrent  ducts  for  the  liberation  of  the  germ 
cells. 

The  conditions  of  this  second  hypothetical  historic  stage' 
are  realized  in  almost  every  detail  by  the  annelid  worms, 
allowing  for  a  few  modifications.  [Cf.  Fig.  139.]  To  some 
this  indicates  the  conclusion  arrived  at  independently  by  the 
consideration  of  other  systems,  that  these  animals  lie  very 
near  the  main  stem  of  vertebrate  ancestry,  but  to  others  this 
is  no  more  than  a  case  of  parallel  development,  in  no  way 
more  remarkable  than  countless  other  adaptive  resemblances, 
such  as  the  instance  of  the  eye  in  cephalopods  and  vertebrates. 
However  this  may  be,  the  example  of  the  annelid  is  most  use- 
ful in  showing  us  that  animals  can  exist  in  precisely  the  con- 
dition of  -  the  hypothetical  form  indicated  by  the  study  of 
vertebrate  embryology  and  constructed  from  the  data  thus 
furnished. 

From  this  second  stage,  which  must  be  very  near  the  actual 
condition  in  the  ancestor  of  modern  vertebrates,  the  final 
type  may  be  reached  by  the  introduction  of  a  few  slight  and 
very  natural  modifications.  The  first  of  these  concerns  the 
metaccelic  sacs  and  consists,  first,  of  the  breaking  down  of  the 
dissepiments  between  the  body  segments,  thus  throwing  all 
the  sacs  of  each  side  into  one,  and  secondly,  a  similar  loss,  at 
least  in  part,  of  the  ventral  mesentery,  making  the  two  lateral 
sacs  confluent  below  the  intestine  and  allowing  this  latter  to 
swing  free  in  the  cavity,  suspended  dorsally. 

Thus,  for  the  first  time  is  reached  a  single  secondary  body 
cavity  or  metaccele  (the  definite  ((  ccelom"),  lined  ivith  peri- 
toneum, which  is  re-fleeted  along  the  mid-dorsal  line  and 


THE   URO-GEXITAL    SYSTEM  369 

furnishes  an  investment  and  suspensory  ligament  for  the  in- 
testine. The  ccclom  is  formed,  as  has  been  shown,  by  the  ex- 
pansion and  later  fusion  of  numerous  pairs  of  gonadic  sacs, 
and  is  thus  an  expanded  gonadic  cavity,  while  the  peritoneum- 
is  identical  with  the  walls  of  a  great  compound  gonadic 
sac.  The  many  pairs  of  gonadic  openings  are  lost  and  appear 
either  as  a  single  pair  (the  pori  abdominalis  of  cyclostomes 
and  selachians)  or  are  entirely  lost  (all  higher  vertebrates). 
Owing  to  this  reduction  either  the  second  method  for  the 
liberation  of  germ-cells  is  employed,  the  utilization  of  ne- 
phridia,  or  else  secondary  ducts  are  developed  to  serve  the 
purpose.  The  proliferating  masses  of  germ-cells  project  from 
the  peritoneal  wall  and  become  suspended  in  band-like  liga- 
ments like  the  mesenteries  of  the  intestine,  the  mesovarium  or 
mesorchium,  and  may  either  remain  in  their  original  loca- 
tion or  become  displaced  and  assume  a  secondary  position. 
Finally  the  nephridia  become  confined  to  a  certain  region  of 
the  body,  where  they  may  form  a  pair  of  single  definite  masses, 
the  kidneys,  the  units  of  which  no  longer  open  externally  by 
independent  openings,  but  become  attached  to  a  common  ex- 
current  duct,  which  opens,  either  independently,  or,  more 
usually,  into  the  terminal  portion  of  the  alimentary  canal. 

Turning  now  from  theory  to  fact,  we  may  take  up  the  uro- 
genital  organs  as  they  actually  exist  in  the  various  vertebrate 
groups,  thus  tracing  a  history  by  means  of  which  the  con- 
dition found  in  Man  may  receive  at  least  a  partial  explanation. 
The  condition  in  the  cyclostomes  is  difficult  to  interpret;  the 
teleosts  seem  not  to  come  into  line  with  the  rest  and  represent 
a  side  branch,  which,  perhaps,  presents  an  independent  so- 
lution of  the  problem ;  but  from  the  selachians  and  certain  ga- 
noids directly  to  the  amphibians,  and  from  them  to  the  Am- 
niota  the  history  is  a  fairly  continuous  one.  For  the  sake  of 
clearness  it  will  be  best  to  consider  separately  the  two  systems 
involved,  beginning  with  the  urinary. 

\Ye  have  already  learned  that  organs  performing  the  same 
function  in  the  different  groups  of  vertebrates  are  not  neces- 
sarily homologous,  and  are  familiar  with  such  phenomena  as 


370  HISTORY   OF   THE    HUMAN    BODY 

the  two  tongues,  the  two  sternums,  the  two  sets  of  ribs  and 
the  possibility  of  two  mouths,  but  here  we  enter  into  a 
greater  complexity,  for  the  history  of  the  urinary  organs  in- 
volves three  kidneys,  pronephros,  mesonephros,  and  metanc- 
phros,  each  with  its  associated  parts,  which  represent  as  many 
successive  dynasties  of  organs  that  have  replaced  one  another. 
In  cases  like  that  of  the  two  respiratory  systems,  where  the 
branchial  system  becomes  replaced  by  the  pulmonary,  many  of 
the  parts  of  the  first  become  employed  by  the  second,  often 
in  quite  a  new  capacity;  but  in  the  present  case  an  added 
element  is  introduced  on  the  part  of  the  neighboring  repro- 
ductive system,  which  not  only  employs  at  times  portions  of 
one  of  the  urinary  systems,  but  retains  them  in  its  service  long 
after  the  system  of  which  it  formed  a  part  has  disappeared. 

The  first,  or  pronephrotic,  system,  appears  in  the  embryo 
of  all  vertebrates;  it  functions  during  the  larval  life  of  some 
fishes  and  amphibians  (Fig.  106),  and  in  a  few  teleosts  per- 
sists as  a  functional  organ  in  the  adult,  but  in  other  fishes  and 
in  all  higher  forms  it  becomes  reduced  to  a  few  rudiments. 
It  thus  strongly  suggests  the  assumption  that  it  once  formed 
the  functional  kidney  in  some  vertebrate  ancestors,  from  which 
it  has  been  inherited.  It  consists  of  a  few  nephridial  tubules, 
strictly  metameric  in  arrangement,  that  is,  a  pair  for  each 
of  several  successive  somites,  situated  very  far  anteriorly, 
often  involving  the  first  of  the  trunk  somites.  The  nephridia 
of  each  side  become  associated  together  to  form  a  single 
kidney,  the  pronephros,  and  enter  a  common  pr one phr otic 
duct,  laterally  placed,  and  opening  either  directly  to  the  ex- 
terior in  the  vicinity  of  the  cloacal  opening  or,  more  usually, 
within  the  cloaca  itself  by  means  of  a  papilla  which  projects 
from  its  dorsal  wall. 

This  duct  is,  for  the  most  part,  like  the  nephridia  them- 
selves, mesodermic  in  origin,  although  in  some  of  the  lower 
forms  the  posterior  portion  arises  from  the  ectoderm,  giving 
to  the  entire  duct  a  double  origin.  This  strange  condition 
may  be  in  part  accounted  for  if  we  consider  that  originally 
there  was  a  larger  number  of  nephridia  and  that  each  opened 


THE   URO-GENITAL    SYSTEM 


by  itself  directly  to  the  exterior.  It  may  then  be  supposed 
that  for  the  better  disposal  of  the  excretory  fluid  the  separate 
openings  became  connected  by  a  groove  which  continued  to 
the  side  of  the  cloacal  orifice  and  deepened  posteriorly  into  a 
trough,  from  which  by  a  further  continuation  of  the  process 
an  internal  tube  would  be  formed, 
opening  either  at  the  margin  of  the 
cloaca  or  jusf  within  it.  The  meso- 
dermic  anterior  portion  may  be  the 
result  of  the  fusion  of  the  outer 
ends  of  the  succesive  nephridia, 
each  one  contributing  that  portion 
belonging  to  its  own  somite. 

Typical  pronephridia  (Fig.  107, 
A),  the  units  of  the  pronephros, 
closely  resemble  the  one  given  in 
the  theoretical  description  above. 
They  possess  at  the  inner  end 
ciliated  nephrostomes  and  show  a 
greater  or  less  tendency  to  coil, 
suggesting  a  former  condition  of 
considerable  physiological  effi- 
ciency. Aside  from  this,  they  show 
the  beginning  of  a  relationship  es- 
sentially vertebrate,  and  carried 
out  in  greater  detail  in  the  meso-  FIG.  106.  Frog  tadpole  with 

and        meta-nephrotic       systems,  fu*ctional  pronephros  and  de- 
1    .     .  .  ~  velopmg  mesonephros.     [After 

namely,  an  association  with  capil-  MARSHALL.] 

lary      blood-VeSSels,      enabling      the       v,  ventricle  of  heart;    *,  truncus; 
....  g,    gill    arteries;    ph,    pharynx;    a, 

nephridia  to  extract  waste  mate-  aorta;  h,  aniage  of  hind  Hmbs; 
rial  directly  from  the  blood.  This  ^nep\nru0ss;;  *'  £™£™^l\  "£ 
association  is  here  very  slight,  Wolffian  duc*- 
and  consists  of  segmentally  arranged  tufts  of  capillaries,  glo- 
meruli,  which  protrude  into  the  ccelomic  cavity  and  form 
rounded  elevations  covered  by  the  peritoneum.  These  are 
located  opposite  the  nephrostomes,  and  the  excretory  fluid, 
which  passes  from  the  glomeruli  to  the  ccelomic  cavity,  is 


372  HISTORY    OF    THE    HUMAN    BODY 

A 


FIG.   107.     Diagrams  illustrating  the  two  forms  of  nephridia  character- 
istic of  pro-  and  meso-nephros,  respectively. 

(A)    Pronephros.     (B)    Mesonephros.      Farther   explanation   in   text. 


THE   URO-GENITAL    SYSTEM  373 

taken  up  by  these  latter  organs.  There  is  no  direct  connec- 
tion between  glomerulus  and  nephridium,  although  in  several 
instances  both  the  elevation  containing  the  former  and  the 
nephrostome  become  included  within  a  recess  of  the  ccelom, 
an  arrangement  which  furthers  the  mutual  action  of  these 
parts.  The  number  of  pairs  of  nephridia  involved  is  usually 
small  (3-4),  but  in  the  Gymnophiona,  in  which  the  prone- 
phros  functions  for  a  considerable  period,  there  may  be  as 
many  as  10-13.  Naturally  the  pronephrotic  system  is  seen 
in  its  most  complete  state  among  the  lower  vertebrates;  in 
Amniota  it  is  often  quite  rudimentary  and  variously  modified. 

The  pronephros,  even  when  best  developed,  possesses  but 
a  temporary  existence  and  becomes  supplanted  by  the  mesone- 
phros,  the  kidney  of  the  second  or  mesonephrotlc  system.  This 
organ  is  formed  from  nephridia  which  are,  like  the  first,  seg- 
mental  in  origin  and  arise  from  somites  posterior  to  those 
associated  with  the  previous  system.  It  forms  the  perma- 
nent kidney  of  fishes  and  amphibians,  and  in  the  embryo  of 
Sauropsida  and  Mammalia  it  is  large  and  prominent  and  has 
been  known  as  the  "  Wolflian  body''  named  in  honor  of  its 
discoverer. 

The  separate  units  of  this  system,  the  mesonephridia  (Fig. 
107,  B),  differ  in  one  essential  particular  from  those  of  the 
pronephros,  namely,  in  their  closer  association  with  the  ar- 
terial glomeruli. 

In  the  case  of  the  pronephridia  these  capillary  tufts  were 
merely  brought  into  close  relation  to  the  nephrostomes,  but 
each  mesonephridium  surrounds  a  glomerulus  with  a  thin- 
walled  evagination  from  its  side,  which  fits  about  it  like  a 
double  cup  and  forms  what  is  known  as  a  Bowman's  capsule. 
The  entire  structure  thus  formed,  including  both  the  capsule 
and  its  glomerulus,  forms  a  renal  [Malpighian]  corpuscle. 
Otherwise  the  mesonephridia  are  like  those  of  the  former 
system,  and  possess  nephrostomes  and  coils.  They  develop 
no  duct  of  their  own  but  utilize  the  pronephrotic  duct,  be- 
coming secondarily  connected  with  it  posterior  to  its  con- 
nection with  the  pronephridia.  Later  on  both  pronephridia 


374  HISTORY   OF    THE    HUMAN    BODY 

and  that  portion  of  the  duct  anterior  to  the  connection  with 
the  mesonephridia  become  atrophied,  and  the  duct  thus  be- 
comes the  mesonephrotic,  the  "  Wolffian  duct "  of  an  earlier 
nomenclature.  On  account  of  this  utilization  of  the  prone- 
phrotic  duct  by  the  mesonephrotic  tubules  it  has  been  held  by 
some  that  both  belong  to  one  system  and  that  the  latter  are 
merely  later  appearing  elements  of  the  pronephrotic  series, 
but  this  is  discredited  by  others  on  the  ground  of  the  differ- 
ence in  the  time  of  functional  activity  of  the  two  systems  and 
also  on  account  of  the  several  somites  without  nephridia  of 
either  kind  that  intervene  between  the  two.  The  structural 
difference  between  the  two  types  of  nephridia,  one  with  a 
Bowman's  capsule,  the  other  without,  may  be  also  employed 
as  an  argument  in  favor  of  the  distinctness  of  the  two  systems, 
but  this  argument  is  weakened  by  a  consideration  of  the  man- 
ner of  formation  of  this  new  part,  and  by  the  assumption  of  the 
existence  of  almost  every  grade  of  transition  between  the  two. 
Thus  in  its  more  usual  form  a  pronephrotic  tubule  is  related 
to  the  accompanying  glomerulus  much  as  in  Fig.  108,  A,  but 
in  some  cases  the  projection  bearing  the  glomerulus  becomes 
partly  enclosed  in  a  recess  of  the  ccelom,  and  the  nephrostome 
opens  into  this  instead  of  into  the  main  cavity  (Fig.  108, 
B).  The  development  of  cilia  at  the  narrow  passage  which 
leads  into  the  recess,  and  the  loss  of  them  around  the  mar- 
gin of  the  original  nephrostome,  would  convert  the  entire 
apparatus  into  a  mesonephrotic  tubule,  in  which  the  added  por- 
tion, including  the  new  nephrostome  and  the  Bowman's 
capsule,  is  a  contribution  from  the  peritoneum.  These  dia- 
grams seem  absolutely  persuasive,  but  unfortunately  do  not 
correspond  with  the  actual  facts,  since,  although  in  special 
cases  the  glomeruli  of  the  pronephrotic  system  are  related 
much  as  in  the  second  diagram,  the  Bowman's  capsules  of  the 
mesonephrotic  tubules  develop  directly  as  evaginations  from 
their  walls,  and  there  is  thus  no  indication  of  either  the  par- 
ticipation of  the  ccelom  or  of  the  formation  of  a  new  nephro- 
stome. 

The  mesonephrotic  system  may  be  found  in  full  functional 


THE   URO-GENITAL    SYSTEM 


375 


activity  in  any  adult  fish  or  amphibian.  Originally  involving 
a  large  number  of  somites  the  mesonephros  extends,  usually  as 
a  pair  of  long,  narrow  organs,  along  a  large  portion  of  the 
trunk,  lying  close  up  against  the  vertebrae  and  ribs.  The 
original  nephridia  become  greatly  multiplied  and  lose  more 
or  less  of  their  original  segmental  arrangement,  the  nephro- 
stomes  appearing  irregularly  along  the  ventral  surface,  that  is, 
the  surface  turned  toward  the  ccelomic  cavity.  The  mesone- 


FIG.  108.  Diagrams  to  illustrate  a  theory  of  the  relationship  of  the 
pro-  and  meso-nephrotic  tubules  to  each  other.  [After  GEGENBAUR.] 

(A)  Stage  of  the  pronephros.  Its  nephrostome  (p)  is  placed  opposite  the 
glomerulus,  but  with  a  small  portion  of  ccelom  interposed  between  them.  (B)  Stage 
of  the  mesonephros.  Here  by  the  formation  of  a  new  nephrostome  at  s  the  inter- 
posed piece  of  the  ccelom  has  become  included  in  the  nephridium,  forming  a  Bow- 
man's capsule. 

phrotic  (Wolffian)  duct  lies  along  its  outer  side  and  sustains 
that  relationship  to  the  cloaca  noted  above  under  the  pro- 
nephrotic  system,  usually  opening  by  a  urinary  papilla  into 
the  dorsal  wall  of  the  cloaca. 

The  third  urinary  system,  the  metane phrotic,  arises  di- 
rectly from  the  second,  and  thus  the  relation  between  the  two 
is  closer  than  that  between  the  second  and  the  first.  It  is  the 
definitive  urinary  system  of  the  amniotes,  and  in  all  reptiles, 
birds  and  mammals  ultimately  replaces  the  mesonephrotic, 
although  this  latter  system  is  well  developed  during  embry- 


376  HISTORY    OF    THE    HUMAN    BODY 

onic  life.  The  metanephrotic  system  is  not  laid  down  at  first 
in  the  form  of  nephridia  as  in  the  other  cases,  but  arises  as  a 
blind  canal  or  evagination  from  the  mesonephrotic  duct  near 
its  lower  end.  This  evagination,  medial  at  its  origin,  comes 
to  lie  dorsal  to  the  mesonephrotic  duct  and  develops  anteriorly 
until  it  comes  in  contact  with  the  dorsal  wall  of  the  ccelomic 
cavity,  where  it  meets  a  mass  of  indifferent  cells  proliferating 
from  it.  From  the  differentiation  of  this  cell  mass  develop 
numerous  nephridia  of  a  type  similar  to  but  in  certain  charac- 
ters distinct  from  either  of  the  other  types,  and  from  the  re- 
peated branching  of  the  anteriorly  growing  canal  there  de- 
velop collecting  tubules  with  which  the  nephridia  unite.  The 
expanded  bases  of  the  terminal  branches  of  the  tube  form  a 
pocket  or  pelvis,  which  collects  the  fluid  from  the  tubules.  The 
nephridia  and  collecting  tubules  form  together  the  definitive 
kidney,  the  metanephros,  while  the  main  tube,  beginning  with 
its  expanded  pelvis,  becomes  the  ureter. 

Each  elementary  unit  of  the  metanephros,  a  metanephri- 
dium,  is  like  that  of  the  previous  system  without  the  nephro- 
stome.  The  connection  with  the  circulatory  system  through 
the  glomeruli,  which  when  first  introduced  was  clearly  a  sec- 
ondary function  of  the  nephridia,  becomes  in  the  mesone- 
phros  of  primary  importance  through  the  development  of  a 
Bowman's  capsule,  and  in  the  metanephros  all  direct  connec- 
tion with  the  ccclom  is-  given  up.  Here,*  in  addition  to  the 
association  with  the  circulatory  system  through  the  Bowman's 
capsules,  the  tubules  themselves  become  very  long  and  at- 
tenuated, and,  as  they  are  accompanied  by  a  rich  network  of 
capillaries,  they  are  enabled  to  extract  the  waste  products 
through  their  entire  length  as  well  as  at  the  localized  renal 
corpuscles. 

This  history  of  the  development  of  the  metanephrotic  sys- 
tem, so  different  in  origin  from  that  of  the  other  two,  and  yet 
so  similar  in  its  results,  has  led  to  much  speculation.  It  can- 
not be  supposed  for  a  moment  that  nephridia  so  nearly  alike 
as  those  of  the  meso-  and  meta-nephros  can  have  developed 
independently,  for  that  would  involve  also  an  independent 


THE    URO-GEXITAL    SYSTEM  377 

origin  in  the  two  cases  of  such  complicated  structures  as  the 
renal  corpuscles.  The  primary  location  of  the  metane- 
phros,  posterior  to  that  of  the  mesonephros,  or  at  least  to  that 
of  its  functional  portion,  leads  to  the  idea  that  the  nephridia 
of  this  system  were  originally  a  part  of  the  mesonephrotic 
series,  belonging  to  its  more  posterior  somites,  and  that  their 
development  from  a  structureless  mass  is  a  case  of  shortened 
development,  in  which  the  primary  segmental  arrangement  has 
become  lost.  The  necessity  for  the  development  of  a  new 
ureter  is  easily  seen  in  the  employment  of  the  older  one  (the 
mesonephrotic  or  Wolffian  duct),  as  a  ductus  [vas]  deferens, 
a  point  to  be  brought  out  later  in  connection  with  the  re- 
productive system. 

The  external  form  of  the  metanephros  varies  considerably. 
This  in  the  Sauropsida  is  in  accordance  with  the  form  of  the 
dorsal  skeletal  wall,  to  which  it  is  closely  applied.  In  struc- 
ture it  is  usually  distinctly  divided  into  lobes  that  correspond 
to  the  terminal  branches  of  the  ureter.  This  is  characteristic 
of  the  kidney  of  most  mammals,  and  the  compact  form  found 
in  Man  is  attained  considerably  after  birth,  and  is  met  with  in 
only  a  few  cases. 

As  may  be  followed  from  the  development,  the  ureters  ter- 
minate posteriorly  in  the  mesonephrotic  ducts  and  may  be  ex- 
pected to  share  the  common  outlet  into  the  cloaca.  This  is 
actually  the  case  in  snakes,  crocodiles  and  birds,  which  con- 
sequently never  perform  urination  as  a  distinct  act,  but  in 
other  reptiles  and  in  mammals  there  is  found  a  terminal 
resevoir,  the  urinary  bladder,  with  which  the  ureters  become 
secondarily  connected.  This  opens  at  first  directly  into  the 
cloaca,  but  its  narrowed  neck  develops  in  the  higher  mammals 
into  a  distinct  canal,  the  urethra,  which  in  the  male  comes  into 
direct  association  with  the  ductus  deferens.  The  urinary  blad- 
der is  no  newr  formation,  but  is  the  remnant  of  the  inner  end 
of  the  allantois,  an  extensive  embryonal  membrane,  which 
passes  out  of  the  body  at  the  umbilicus  and  becomes  in  the 
Sauropsida  an  external  respiratory  organ,  and  in  mammals 
furnishes  the  essential  parts  of  the  umbilical  cord  and  placenta. 


378  HISTORY   OF   THE    HUMAN    BODY 

After  birth  a  portion  of  this  becomes  shut  within  the  body  by 
the  closure  of  the  umbilical  connection,  and  as  this  portion  is 
in  the  form  of  an  open  bag  leading  out  from  the  cloaca,  it  is 
easily  converted  into  a  reservoir  for  urine,  the  greatest  change 
necessary  being  a  slight  shifting  of  the  terminal  portions  of 
the  ureters.  Only  the  lower  portion  is  actually  utilized  for  this 
purpose,  and  the  remainder  atrophies  into  a  ligament,  which 
extends  from  the  apex  of  the  bladder  to  the  umbilicus.  Ap- 
proaching the  cloaca  the  bladder  becomes  narrowed  to  a 
small  neck  which  is  continued  as  a  median  duct  or  canal,  the 
urethra,  and  opens,  in  common  with  the  genital  ducts,  into 
the  urogenital  sinus. 

A  structure  called  a  urinary  bladder  is  present  in  amphib- 
ians. This  is  in  the  form  of  a  collapsed  bag  leading  out  from 
the  ventral  wall  of  the  cloaca  and  is  without  direct  connection 
with  the  urinary  system.  This  seems  to  represent  morpholog- 
ically an  undeveloped  allantois,  and  is  thus  really  homologous 
with  the  bladder  of  the  Amniotes.  Its  function  is  not  wholly 
understood,  as  it  never  appears  to  contain  liquid,  but  the  occa- 
sional presence  within  it  of  excretory  salts  suggests  a  sub- 
ordinate use  in  connection  with  the  urinary  system. 

The  second  of  the  two  associated  systems  to  be  considered 
is  that  of  reproduction  (generation),  and  consists  primarily 
o£  the  germ  glands,  in  vertebrates  a  single  pair,  together  with 
some  definite  avenue  of  escape  for  the  mature  germ  cells.  To 
these  may  be  added  secondarily  external  parts  to  insure  the 
union  of  the  two  sorts  of  germ  cells. 

The  germ  glands,  the  essential  organs  of  reproduction^ 
develop  as  localized  areas  on  the  peritoneal  wall  of  the  ccelom, 
and  are  primarily  located  dorsally,  one  on  either  side  of  the 
vertebral  column,  in  about  the  middle  of  the  trunk  region. 
This  similar  origin,  from  avlayer  which  otherwise  forms  noth- 
ing but  investing  membranes  and  suspensory  ligaments,  is 
easily  explained  by  the  theory  given  above,  which  considers 
the  entire  ccelom  as  the  result  of  the  fusion  of  a  series  of  ex- 
panded gonads,  a  theory  perfectly  in  harmony  with  all  the 
related  facts.  The  germ  glands,  primarily  patches  of  germinal 


THE   URO-GENITAL    SYSTEM  379 

epithelium,  become  reinforced  from  behind  by  the  prolifera- 
tion of  connective  tissue,  containing  nerves  and  blood-vessels ; 
and  thus  are  formed  mounds  projecting  into  the  ccelom,  cov- 
ered with  germ  cells.  This  association  becomes  more  intimate 
through  the  intrusion  of  the  germinal  epithelium  into  the  in- 
terior in  many  places,  where  the  cells  receive  the  nourishment 
necessary  for  their  complete  development. 

The  germ  cells  themselves  are  of  two  sorts,  ova  and  sper- 
matozoa, and  their  differences  in  form  and  size  necessitate  a 
more  or  less  apparent  difference  in  the  organs  that  produce 
them,  the  ovaries  and  testes  respectively.  In  certain  cases 
among  cyclostomes  the  same  germ  gland  produces  both  sorts 
of  germ  cells,  although  at  different  times,  but  with  this  exr 
ception  the  sexes  are  normally  separate.  The  occasional  oc- 
currence of  a  few  cells  of  one  sort  in  a  gland  which  normally 
produces  the  other,  as  the  development  of  a  few  ova  on  the 
side  of  a  testes,  or  vice  versa,  occurs  as  an  anomaly  among 
many  of  the  lower  vertebrates,  and  this  phenomenon,  taken 
in  connection  with  the  cases  among  the  cyclostomes  just  cited, 
has  led  to  the  possible  theory  that  the  ancestors  of  vertebrates 
were  hermaphroditic,  as  is  the  case  in  many  invertebrates, 
but  there  is  little  else  to  indicate  this.  Reported  hermaphro- 
dites among  higher  vertebrates  are  usually  if  not  always  ap- 
parent rather  than  real  and  are  in  fact  malformations  due  to 
some  error  in  development  affecting  mainly  the  external  parts. 

All  that  is  essential  for  the  production  of  a  new  organism  is 
the  complete  and  intimate  union  of  the  two  germ  cells,  one 
of  each  sort,  but  the  varied  environment  of  the  parents  often 
makes  it  a  problem  to  arrange  the  means  by  which  this  may 
be  accomplished.  It  offers  the  least  difficulty  in  the  case  of 
aquatic  forms,  for  all  that  is  here  necessary  is  to  liberate  the 
cells  of  both  sorts  into  the  water,  in  which  the  union  can  be 
easily  effected,  since  the  water  furnishes  the  fluid jmedium  nec- 
essary for  the  locomotion  of  the  spermatozoa.  Often,  too, 
such  animals  associate  in  pairs  and  develop  elaborate  instincts 
which  insure  the  discharge  of  the  two  products  in  close  prox- 
imity to  one  another. 


380  HISTORY   OF   THE    HUMAN    BODY 

This  absolute  necessity  of  a  fluid  medium  causes  the  de- 
velopment in  land  forms  of  a  number  of  accessory  parts. 
Thus  there  develop  in  the  male  special  glands  to  supply  a  ve- 
hicle for  the  spermatozoa,  forming  a  spermatic  fluid;  and  as 
this  cannot  be  allowed  to  dry  up,  it  must  be  conveyed  directly 
to  the  female  by  an  internal  copulation,  necessitating  again- 
certain  modifications  of  the  cloacal  margin,  from  which  de- 
velop the  various  external  organs.  Although  it  is  evident  that 
the  development  of  the  process  of  copulation  is  here  due  solely 
to  the  terrestrial  life,  there  are  sometimes  other  conditions 
that  develop  it,  for  although,  on  the  one  hand,  it  is  universal 
among  terrestrial  forms,  invertebrates  as  well  as  vertebrates, 
it  is  occasionally  found  among  aquatic  animals,  notably  in  tliis 
connection  the  selachians;  that,  however,  it  is  here  an  inde- 
pendent development  is  shown  by  the  source  from  which  the 
copulatory  organs  are  derived,  namely,  from  the  inner  margin 
of  the  ventral  fins,  and  not  from  the  rim  of  the  cloaca,  as  in 
higher  vertebrates. 

There  are  thus  two  groups  of  accessory  reproductive  organs 
to  be  considered,  (i)  those  which  furnish  an  outlet  for  the 
germ  cells,  and  (2)  those  which  are  concerned  in  internal 
copulation.  These  may  be  taken  up' in  order. 

The  conception  of  the  peritoneal  cavity  as  an  expanded  go- 
nadic  sac  demands  that  the  germ  cells  generated  in  its  wall 
should  break  loose  and  float  about  within  the  ccelom,  until 
finally  expelled  either  through  some  direct  channel  of  com- 
munication with  the  exterior,  the  original  gonadic  ducts,  or 
else  by  utilization  of  some  part  of  the  nephridial  system;  and 
as  a  matter  of  fact  all  conditions  found  in  vertebrates,  with 
the  possible  exception  of  that  of  teleosts,  may  be  directly  re- 
ferred to  one  of  these  methods. 

The  most  primitive  condition  is  that  seen  in  cyclostomes,  in 
which  the  peritoneal  cavity  communicates  directly  with  the 
.exterior  by  means  of  a  pair  of  pori  abdominales,  canals  which 
begin  at  the  posterior  part  of  the  ccelom  and  open  along  the 
sides  of  the  cloacal  orifice.  The  germ  cells,  when  matured, 
become  freed  from  their  place  of  origin  and  float  about  in  the 


THE   URO-GENITAL    SYSTEM  •      381 

peritoneal  cavity  until  discharged  through  these  abdominal 
pores.  The  urinary  organs  have  no  direct  connection  with 
this  system  other  than  through  the  nephrostomes  which  open 
into  the  peritoneal  cavity,  and  these  are  not  specialized  to  re- 
ceive the  free  germ  cells. 

There  is  thus  shown  the  original  condition  of  gonads  and 
their  excurrent  ducts,  slightly  modified  by  the  fusion  of  all  the 
gonads  into  one  and  the  reduction  of  the  gonadic  ducts  to  a 
single  pair.  Otherwise  the  primitive  physiological  functions 
are  carried  on  as  they  were  before  the  gonadic  cavities  became 
converted  into  a  metaca?le. 

It  will  be  noticed  that  in  the  above  description  the  ne- 
phrostomes open  directly  into  the  ccelomic  cavity  and  thus 
suggest  the  possibility  of  the  use  of  nephridia  for  the  exit  of 
the  germ  cells.  Such  is  actually  the  next  stage  in  the  history 
of  these  organs,  for  in  the  selachians  certain  of  the  nephridia 
are  so  employed  while  the  pori  abdominales,  although  they 
still  exist,  are  no  longer  used  for  their  original  purpose.  In 
the  male  the  testes  lie  in  close  proximity  to  the  anterior  por- 
tion of  the  kidneys,  and  enter  into  direct  connection  with  the 
nephridia  of  this  region  through  the  development  of  a  series 
of  tubes,  the  vasa  efferentia,  which  extend  from  the  testes  and 
enter  the  nephridia  a  little  beyond  the  nephrostomes.  The 
original  function  of  this  part  of  the  kidneys  is  not  impaired, 
and  during  the  greater  part  of  the  time  it  exercises  the  urinary 
function  alone;  but  during  the  periods  of  sexual  activity  the 
nephridia  involved  become  filled  with  the  spermatic  fluid  and 
deliver  it  directly  from  the  testes  to  the  mesonephrotic  duct 
and  thence  to  the  cloaca.  From  there  it  is  received  into  a 
channel  formed  by  the  approximation  of  the  inner  modified 
portions  of  the  ventral  fins,  and  delivered  within  the  cloaca  \ 
of  the  female  by  an  internal  copulation,  an  unusual  method / 
among  aquatic  animals.  That  there  is  no  genetic  connection 
between  this  act  and  that  developed  among  terrestrial  verte- 
brates may  be  seen  from  the  employment  of  very  different 
organs  for  the  purpose  in  the  two  cases  and  from  the  fact  of 
the  interposition,  in  the  direct  line  of  descent,  of  forms  that 


382  HISTORY   OF   THE    HUMAN    BODY 

do  not  develop  any  such  method.  Through  this  close  con- 
nection between  the  originally  distinct  reproductive  and  uri- 
nary systems  it  results  that  both  the  anterior  part  of  the  meso- 
nefrhros  and  the  mesonephrotic  duct  become,  apart  from  their 
urinary  function,  accessory  reproductive  organs,  the  former 
serving  as  a  "  sexual  kidney,"  and  the  latter  as  a  ductus 
deferens,  or  excurrent  seminal  duct. 

In  the  female  selachian  a  different  modification  takes  place, 
seemingly  not  due  to  association  with  the  urinary  system,  but 
proven  to  be  so  by  the  developmental  history  of  the  parts. 
The  ovary  of  the  adult  occupies  about  the  same  position  as  do 
the  testes  of  the  male,  but  shows  no  direct  connection  with  the 
anterior  part  of  the  kidney.  In  place  of  this  there  appears  on 
each  side  a  long  tube  running  along  the  side  of  the  mesone- 
phrotic duct  and  opening  posteriorly  into  the  cloaca  beside 
that  of  its  associate.  This  is  the  oviduct,  or  "  Midler's  duct " 
of  many  writers.  At  the  free  anterior  end,  which  extends  to 
almost  the  forward  limits  of  the  ccelom,  it  opens  by  an  ex- 
panded mouth,  ostium  tubes,  directly  into  this  latter  cavity 
and  receives  into  this  the  mature  ova  which  become  released 
from  the  ovary  and  wander  about  in  the  ccelom  in  the  primitive 
fashion.  The  oviduct  arises  in  the  embryo  as  a  tube  seg- 
mented off  longitudinally  from  the  mesonephrotic  duct  by  the 
common  method  of  the  development  of  two  longitudinal  folds 
opposite  one  another,  and  thus  points  to  a  period  at  which 
the  ova  as  well  as  the  spermatozoa  were  conveyed  to  the  cloaca 
through  the  mesonephrotic  duct.  The  ostium  is  probably  an 
enlarged  and  specialized  nephrostome,  associated  with  a  single 
nephridium,*  and  it  is  thus  easily  imagined  that  the  primary 
conditions  in  the  female  corresponded  closely  to  that  of  the 
male,  but,  that  owing  to  the  greater  size  of  the  products  to  be 


*The  not  infrequent  occurrence,  even  in  the  human  subject,  of  two 
ostia  upon  one  side  may  possibly  be  the  result  of  the  retention  of  two 
nephrostomes  instead  of  a  single  one,  or  it  may  be  simply  an  anomaly 
like  the  multiplication  of  digits  on  other  parts.  If  it  be  the  first  it  con- 
cerns a  very  ancient  bit  of  history,  and  suggests  an  extreme  degree  of 
reversion. 


THE   URO-GENITAL    SYSTEM  383 

transmitted,  a  single  nephridium  with  its  nephrostome  became 
differentiated  for  this  purpose,  and  that  later  on  there  came  a 
longitudinal  splitting  of  the  primary  mesonephrotic  duct,  be- 
ginning above  and  progressing  gradually,  for  the  better  ac- 
commodation of  the  sexual  products.  The  employment  of  a 
nephrostome  instead  of  vasa  efferentia  is  quite  a  fundamental 
difference,  but  rudiments  of  these  latter  vessels  are  to  be  de- 
tected in  association  with  the  ovaries,  and  thus  the  use  of  the 
former  may  have  been  a  later  adaptation. 

The  uro-genital  relations  of  the  selachians  seem  to  have  been 
inherited  directly  by  the  amphibians  (Fig.  109,  a  and  c),  for 
the  two  correspond  closely;  in  the  male  there  is  the  same  re- 
lationship between  testes  and  sexual  kidney,  and  the  meso- 
nephrotic duct  is  a  common  ureter  and  ductus  deferens.  A 
rudimentary  oviduct  tapering  anteriorly  to  a  blind  end,  is  usu- 
ally found  attached  to  the  side  of  this  latter  tube.  In  the  fe- 
male the  oviduct  is  often  very  long  and  convoluted,  and  its 
walls  are  often  glandular  and  furnish  membranous  and  gelat- 
inous encasements  for  the  eggs.  In  a  few  instances  the  lower 
part  of  the  tube  is  expanded  into  a  uterus  for  the  retention 
of  the  larva. 

Corresponding  to  the  lack  of  internal  copulation  there  are 
no  external  organs,  but  there  are  various  instincts  developed 
which  have  for  their  purpose  the  mingling  of  the  sexual 
products.  Thus  the  males  of  some  aquatic  salamanders  pro- 
duce conical  spermatophores,  which  rest  upon  the  sand  at  the 
bottom  of  the  pond  and  are  taken  up  by  the  cloaca  of  the  fe- 
male ;  a  similar  purpose  is  seen  in  the  amplexation  of  frogs  and 
toads,  in  which  the  males  embrace  the  females  during  ovi- 
position  and  void  the  seminal  fluid  over  the  egg  masses  as  soon 
as  laid. 

A  fundamental  change  of  relationship  is  seen  in  the  Am- 
niota,  caused  by  the  appearance  of  the  third  kidney,  the  meta- 
nephros.  This  organ,  which  possesses  a  separate  ureter  and 
is  thus  a  complete  urinary  system  in  itself,'  assumes  the  entire 
control  of  this  function,  and  leaves  to  its  predecessor,  the 
mesonephrotic  system,  nothing  but  reproductive  functions. 


384 


HISTORY    OF    THE    HUMAN    BODY 


A 


FIG.     109.     Comparison    of    the    urogenital    system    in    Anamnia    and 
Ammota.     (Continued  at  bottom  of  p.  385.) 


THE   URO-GENITAL    SYSTEM  385 

As  a  result  of  this,  those  parts  of  this  latter  system  which  have 
been  previously  employed  for  reproductive  purposes  are  re- 
tained and  even  become  more  highly  specialised,  while  the  parts 
that  were  wholly  urinary  disappear,  with  the  exception  of  a 
few  vestiges. 

In  this  the  two  sexes  are  affected  differently,  as  may  be  made 
clear  by  a  reference  to  Fig.  109,  in  which  a  and  b  show  the 
changes  produced  in  the  male,  c  and  d  those  in  the  female. 
In  the  male  amphibian  the  sexual  parts^are  the  testes,  the  vasa 
efferentia  and  the  mesonephrotic  duct;  in  the  male  amniote 
these  parts  are  retained  while  the  remainder  of  the  mesone- 
phrotic system  has  disappeared,  being  replaced  by  the  meta- 
nephrotic.  The  mesonephrotic  duct,  released  from  all  urinary 
function,  becomes  the  definite  ductus  deferens,  and  the  re- 
maining portion  of  this  system,  including  vasa  efferentia, 
sexual  kidney  ana  collecting  efferent  tubules,  becomes  closely 
associated  with  the  testes  under  the  name  of  the  epididymis.  In 
the  female  amphibian  the  reproductive  system  has  become  prac- 
tically independent  of  the  urinary  through  the  development  of  a 
separate  excurrent  duct,  the  oviduct,  and  thus,  with  the  rise  of 
the  metanephrotic  system,  that  of  the  mesonephros  becomes 
reduced  to  a  few  functionless  vestiges ;  yet  the  more  conserva- 
tive embryonic  history  records  the  fact  that  both  oviduct  and 
ostium  were  originally  portions  of  the  mesonephrotic  system, 
and,  although  with  a  different  history,  both  sexes  are  in  reality 
about  equally  indebted  to  it  for  their  accessory  organs. 

Although  the  reproductive  organs,  as  given  in  the  above 
sketch,  are  the  common  heritage  of  all  amniotes,  the  separate 
groups  of  reptiles,  birds,  and  mammals  have  been  left  to  work 
out  the  details  in  accordance  with  their  own  necessities.  In 
each  there  is  a  metanephrotic  urinary  system,  with  kidneys 
and  ureters  distinct  from  the  reproductive  system  except  for 
intimate  topographical  relationships  at  their  outlets;  in  the 


(A)  Male  anamnian.  (B)  Male  amniote.  (C)  Female  anamnian.  (D)  Female 
amniote. 

t,  testis;  o,  ovary;  ms,  mesonephros;  c,  that  part  of  the  mesonephros  which  is 
associated  with  the  germ  gland,  (in  male  amniotes  this  becomes  the  epididymis); 
t»,  Wolffian  duct  (ductus  deferens);  in,  Miiller's  duct  (oviduct;  mu,  uterus);  k, 
metanephros;  b,  bladder  (of  the  Amniota) ;  n,  ureter;  r,  rectum;  v,  vagina. 


386          .   HISTORY    OF    THE    HUMAN    BODY 

male  the  testes  are  accompanied  by  an  epididymis  and  a  ductus 
deferens,  respectively  the  anterior  portion  of  the  mesonephros 
and  the  mesonephrotic  duct;  and  in  the  female  there  is  an 
oviduct  with  an  enlarged  ostium,  into  which  the  wandering 
ova  are  received.  In  the  present  treatise  the  details  of  these 
parts  in  reptiles  and  birds  cannot  be  considered  further,  but  the 
history  that  is  shown  in  mammals  is  of  much  importance,  as 
it  includes  the  human  conditions. 

In  the  mammalian  embryo  the  mesonephrotic  system  at- 
tains a  high  degree  of  development,  and  the  mesonephros, 
under  the  name  of  the  "  Wolffian  body,"  is  large  and  con- 
spicuous. In  the  marsupial  young  of  monotremes  and  mar- 
supials it  forms  the  functional  kidney,  and  as  this  is  but  one 
of  several  organs  that  become  profoundly  modified  or  replaced 
during  later  life,  the  development  may  be  rightly  considered  a 
true  metamorphosis  in  which  the  marsupial  young  represent  a 
larval  stage.  In  placental  mammals  a  similar  replacement  of 
urinary  systems  takes  place,  but  as  the  intra-uterine  life  is  here 
made  longer  than  in  former  cases  and  includes  also  approxi- 
mately the  period  passed  by  lower  mammals  in  the  marsupial 
pouch,  there  is  no  free  larva,  and  the  changes  are  considered 
a  part  of  the  embryonic  development. 

As  both  the  stage  of  the  functional  mesonephros  and  its 
later  reduction  are  of  importance  in  understanding  the  adult 
conditions,  they  may  be  first  studied  by  the  aid  of  the  accom- 
panying Plate.  During  the  period  designated  as  that  of  sexual 
indifference,  which  includes  all  the  early  development  and 
continues  until  the  embryo  is  quite  well  matured  in  many 
other  particulars  (up  to  70  or  80  daysJn  the  human  species), 
the  sexes,  although  definitely  determined,  show  absolutely  no 
difference  in  the  general  appearance  of  the  uro-genital  organs. 
[Plate  III,  a].  The  mesonephros  is  large  and  functional  and 
stands  out  freely  from  the  dorsal  wall  of  the  abdomen,  held 
in  place  by  a  suspensory  fold  of  peritoneum,  the  mesonephrotic 
ligament.  This  fold  becomes  prolonged  posteriorly  beyond 
the  limits  of  the  Wolffian  body  and  forms  the  inguinal  liga- 
ment, a  part  of  great  importance  in  subsequent  relationships. 


C     rt 

2  a 

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o 


-H*  e 


w   o 


THE    URO-GENITAL    SYSTEM  387 

Upon  the  inner  side  of  each  mesonephros  appears  a  longitudi- 
nal fold  of  its  peritoneal  investment,  along  the  free  edge  of 
which  the  cells  become  proliferated  and  form  the  germ  gland ; 
the  remaining,  or  basal  portion  of  this  fold  is  later  to  form 
the  mesorchium  or  mesovarium,  the  suspensory  ligament  of 
the  mature  testis  or  ovary  respectively.  The  Wolffian  (meso- 
nephrotic)  duct  lies  along  the  free  edge  of  the  mesonephros, 
and  not  far  from  this  is  Muller's  duct,  suspended  in  a  fold 
which  projects  from  the  ventral  surface  of  the  mesonephros. 
These  two  pairs  of  ducts  are  brought  together  at  their  distal 
ends  and  form  a  common  chamber,  into  which  all  four  empty, 
the  uro-genital  sinus. 

From  this  stage  the  conditions  found  in  the  female  are 
readily  developed.  [Plate  III,  c.]  Its  most  important  organs 
are  the  germ  gland,  which  becomes  the  ovary,  and  Muller's 
duct,  the  upper  part  of  which  becomes  the  oviduct  [uterine 
(Fallopian)  tube],  and  the  lower  part,  the  uterus.  The  en- 
tire mesonephrotic  system,  since  it  is  in  no  wise  concerned  in 
the  reproductive  function  and  since  the  urinary  function  is 
wholly  assumed  by  the  metanephrotic  system,  disappears  ex- 
cept for  a  few  useless  vestiges;  its  loss  allows  the  mesone- 
phrotic ligament  to  become  continuous  with  that  of  Muller's 
duct,  and  thus  to  extend  as  the  broad  ligament  of  oviduct  and 
uterus  from  the  dorsal  body  wall  to  the  latter  organs.  The 
round  ligament  is  formed  from  the  posterior  extension  of  this 
latter,  the  ligamentum  inguinale. 

Muller's  duct  gives  rise  to  the  oviduct,  uterus  and  vagina, 
which  are  thus  seen  to  be  nothing  more  than  differentiations 
of  the  various  regions  of  a  single  tube.  The  ostium  is  much 
nearer  approximated  to  the  ovary  than  in  Sauropsida,  and  as 
it  opens  and  partly  surrounds  the  latter  during  ovulation,  the 
entrance  of  the  ova  into  the  oviduct  is  practically  assured. 
In  some  mammals  there  is  a  special  arrangement  in  the  form 
of  a  recess  or  pocket  of  peritoneum,  the  bursa  ovarica,  in  which 
the  ovary  lies,  covered  by  the  ostium,  and  in  a  few  cases  the 
fusion  of  the  edges  of  ostium  and  bursa  convert  the  latter  into 
a  capsule  which  may  either  open  to  the  ccelom  through  a  small 


388  HISTORY   OF   THE    HUMAN    BODY 

foramen  or  may  be  absolutely  closed.  In  the  primates,  in- 
cluding Man,  the  connection  is  not  as  intimate  as  this,  and  the 
ova  occasionally  escape  into  the  ccelomic  cavity,  as  is  normally 
the  case  among  lower  forms.  Here,  however,  they  usually 
disintegrate  and  become  lost,  but  in  rare  cases  a  fertilized 
egg  escapes  in  this  way  and  may  even  attain  considerable  de- 
velopment through  the  formation  of  a  sort  of  placenta,  at- 
tached to  the  ccelomic  wall. 

A  uterus  is  more  or  less  an  adaptive  organ,  related  to 
Miiller's  duct  much  as  the  crop  is  to  the  oesophagus;  it  is 
primarily  nothing  but  a  localized  enlargement  and  develops 
whenever  needed,  in  some  cases  appearing  in  a  given  form, 
while  absent  in  a  closely  related  one.  Thus  in  vivaparous 
sharks  (e.g.,  Squalus),  the  expanded  lower  portion  of  each 
Miiller's  duct  becomes  enlarged  and  forms  a  uterus  in  which 
the  embryos  are  retained  until  they  reach  practically  the  adult 
form;  and  the  same  is  true  in  the  case  of  a  certain  salamander 
(Salamandra  atra).  In  none  of  these  cases,  however,  is  the 
organ  more  than  a  container  or  brood  cavity,  and  there  is  no 
placenta  formation  or  other  direct  connection  between  embryo 
and  uterine  wall.  The  same  is  true  of  the  lower  mammals, 
the  marsupials  and  monotremes,  in  which  there  is  no  placenta, 
and  the  young  are  produced  in  a  very  immature  state ;  but  in 
the  higher,  or  placenta!,  mammals,  the  wall  of  the  uterus 
becomes  differentiated  for  the  purpose  of  the  nutrition  of  the 
embryo,  and  thus  becomes  a  definite  physiological  organ. 

There  are  numerous  types  of  uterus  among  the  mammals, 
depending  on  the  degree  of  fusion  between  the  Miiller's  ducts 
of  the  two  opposite  sides ;  and  these  types  consequently  pre- 
sent a  regular  graded  series  between  two  distinct  lateral  uteri 
and  a  single  median  one  (Fig.  no). 

In  the  first  type  of  this  series,  that  seen  in  monotremes,  the 
two  ducts  are  entirely  distinct  from  one  another.  They  are 
short,  thick  walled,  and  of  rather  large  caliber,  and  may  be 
termed  oviducts  or  uteri,  according  to  the  taste  of  the  writer, 
although  the  former  term  is  more  usually  applied  to  them. 
They  open  separately  into  a  common  urogenital  sinus  in  close 


THE   URO-GENITAL    SYSTEM 


389 


association  with  the  openings  of  the  ureters.  There  are  no 
vaginae,  although  the  urogenital  sinus  serves  functionally 
as  one.  In  the  oviducts  the  small  oval  eggs,  3.5  x  4.0  mm.  in 
diameter  when  they  leave  the  ovary,  are  retained  for  some 
time  and  increase  in  size  to  about  12x15  mm.  through  the 
absorption  of  nutrient  fluids  secreted  by  the  oviducal  walls. 

The  next  stage  is  that  of  the  marsupials,  in  which  the  two 
ducts  are  still  distinct,  but  each  shows  a  differentiation  into 
oviduct,  uterus  and  vagina.  The  two  vaginal  orifices  open 


DUPLEX 


BIPARTITUS 


SIMPLEX 


FIG.  no.    Different  types  of  mammalian  uterus:  explanation  in  text. 

into  a  common  urogenital  sinus,  which  is  here  prolonged  into 
a  canal,  and  opens  to  the  exterior  separately  from  the  rectum. 
Here  for  the  first  time  the  complete  separation  of  these  two 
canals,  urogenital  and  alimentary,  is  met  with,  since  in  mono- 
tremes,  although  internally  distinct,  there  is  externally  a  com- 
mon cloacal  orifice.  The  partition  separating  the  two  becomes 
thicker  and  of  more  importance  in  the  higher  mammals,  and 
forms  the  perin&um. 
Above  the  marsupials  the  two  ducts  fuse  into  one  at  their 


390  HISTORY   OF   THE    HUMAN    BODY 

terminal  portions,  and  form  a  single  vagina,  though  usually 
with  two  lateral  uteri,  various  stages  of  the  fusion  of  which 
form  the  successive  steps  known  as  uterus  duplex,  uterus  bipar- 
titus,  and  uterus  bicornis.  The  first  of  these  types  possesses 
two  distinct  openings  for  the  uteri  (ora  uteri),  which  open 
into  a  common  vagina;  in  the  others  there  is  a  single  os,  but 
two  uterine  compartments.  In  all  cases  the  vagina  is  single, 
but  an  indication  of  its  former  duplicity  is  observed  in  a  few 
animals  (e.g.,  Equus),  in  the  form  of  a  median  longitudinal 
fold. 

The  'duplex  type  occurs  in  Procavia  (Hyrax),  an  iso- 
lated group  of  small  mammals  found  in  Western  Asia  and  in 
Africa;  the  bicornis  is  widely  distributed  and  occurs  in  ungu- 
lates, cetaceans,  most  bats,  and  other  forms;  the  bipartitus 
occurs  in  carnivores,  the  pig,  and  a  few  bats. 

When  several  embryos  are  developed  simultaneously,  as  in 
most  small  mammals,  the  two  uterine  halves  become  drawn 
out  into  long  tubes,  and  the  embryos  are  fixed  at  approximately 
equal  intervals,  each  half  containing  about  the  same  number. 
As  the  embryos  develop,  the  portions  of  the  tube  in  which  they 
lie  become  greatly  enlarged,  while  the  intervening  parts  are 
restricted,  giving  the  whole  the  appearance  of  a  necklace  or 
a  string  of  sausages. 

The  uterus  simplex,  characteristic  of  man  and  the  apes, 
represents  the  extreme  degree  of  fusion  of  the  two  parts.  In 
this  nearly  all  signs  of  its  double  origin  are  lost  and  the 
uterus  assumes  the  form  of  a  balloon-shaped  or  piriform  or- 
gan, somewhat  flattened  dorso-ventrally,  and  possessing  two 
oviducts,  which  open  at  the  slightly  prolonged  antero-lateral 
angles. 

Uterus  and  oviducts  are  supported  in  all  mammals  by  two 
principal  suspensory  ligaments,  the  broad  and  the  round  (ligg. 
latum  et  teres),  which  are  easily  explained  by  comparison  with 
the  indifferent  condition.  [Plate  III,  a.]  Here  the  two  layers 
of  peritoneum  that  form  the  mesonephrotic  ligament  contain 
between  them  the  mesonephros  and  its  duct,  and  become  con- 
tinued along  the  ventral  surface  of  the  mesonephros  as  a  low 


THE   URO-GENITAL    SYSTEM  391 

longitudinal  fold  which  contains  in  its  margin  the  Mullerian 
duct.  The  germ  gland  (here  the  ovary)  is  attached  to  this 
along  the  inner  side  of  the  mesonephros  by  means  of  a  narrow 
mesovarium.  Imagine  now  the  two  important  changes  which 
actually  occur,  namely,  the  complete  reduction  of  the  meso- 
nephrotic  system  and  the  development  of  the  Mullerian  duct 
into  oviduct  and  uterus,  and  we  have  as  a  result  the  broad 
ligament,  arising  from  the  body  wall  and  extending  to  the 
oviduct,  which  it  enwraps  along  its  free  edge.  The  round 
ligament  is  formed  from  the  posterior  slip  of  the  original  meso- 
nephrotic  ligament,  the  ligamentum  ingninale. 

The  vestiges  of  the  mesonephrotic  system  are  found  ex- 
actly where  they  would  be  expected,  lying  in  the  broad  liga- 
ment not  far  from  the  oviduct,  bet\veen  it  and  the  insertion  of 
the  mesovarium.  They  consist  of  three  portions,  epoophoron, 
its  longitudinal  duct,  and  paroophoron.  [Plate  III,  cJ]  The 
first  of  these  ("  organ  of  Rosenmiiller  ")  consists  of  a  series 
of  blind  tubules  attached  to  a  common  duct,  and  plainly  repre- 
sents the  vasa  efTerentia  and  the  upper  portion  of  the  Wolffian 
duct,  in  other  words,  the  "  sexual  kidney."  Below  this  are  a 
few  scattered  tubules,  forming  the  paroophoron  and  represent- 
ing the  lower  or  urinary  portion  of  the  mesonephros.  The 
longitudinal  duct  of  the  epoophoron  ("Gartner's  duct")  is 
the  remnant  of  the  main  part  of  the  Wolffian  duct,  and  lies 
imbedded  in  the  wall  of  the  uterus;  it  occasionally  joins  its 
upper  part  and  thus  completes  the  representation  of  the  meso- 
nephrotic system.  It  occurs  quite  regularly  in  the  pig,  the 
horse  and  in  ruminants,  but  is  only  occasional  in  Man. 

The  original  direction  of  the  mesonephrotic  ligament,  that 
is,  the  direction  which  it  has  in  the  embryo,  and  that  which  is 
retained  in  adult  Sauropsida,  becomes  changed  in  mammals 
and  comes  to  lie  transversally  across  the- dorsal  wall  as  though 
laid  over  laterally  from  above,  the  lower  part  remaining  as  at 
first.  The  principal  effect  of  this  is  to  remove  the  ovaries, 
and  with  them  the  oviducts,  from  their  primary  position  in 
the  lumbar  region  and  located  them  near  or  within  the  brim 
of  the  pelvis,  not  far  from  the  inguinal  region.  This  occa- 


392  HISTORY   OF   THE   HUMAN   BODY 

sions  several  compensatory  changes,  such  as  the  lengthening 
of  the  ovarian  nerves  and  blood-vessels,  which  are  con- 
tained in  the  mesovarium.  The  significance  of  this  process, 
known  from  its  principal  feature  as  the  decensus  ovarioruin, 
is  unknown,  but  it  seems  to  correspond  in  part  to  a  somewhat 
similar  but  more  extensive  descensus  of  the  testis,  found  in 
the  male. 

To  comprehend  the  relationships  of  the  male  organs  in 
mammals,  it  is  best  to  begin  again  at  the  indifferent  stage 
[Plate  III,  a],  which  is  thus  seen  to  furnish  the  starting-point 
for  the  explanation  of  the  reproductive  organs  in  both  sexes. 
While  the  accessory  organs  in  the  female  are  mainly  the  pro- 
duct of  differentiation  in  the  Miillerian  duct,  the  Wolffian  duct 
becoming  vestigial,  in  the  male  it  is  the  Wolffian  duct  that  is 
emphasized,  together  with  the  upper  portion  of  the  meso- 
nephros,  while  the  Miillerian  duct  is  reduced  to  a  few  rudi- 
ments. [Plate  III,  &.]  As  in  male  selachians  and  amphibians, 
the  anterior  tubules  of  the  mesonephros  serve  as  vasa  efferentia 
for  the  conduction  of  the  spermatozoa,  but  here  they  are  used 
exclusively  for  this  purpose,  while  the  nephrostomes,  Bow- 
man's capsules,  and  all  parts  of  those  tubules  once  associated 
with  the  urinary  function,  are  no  longer  developed.  The 
tubules  are  much  convoluted  and  form  a  compact  mass,  closely 
associated  with  the  testis,  the  epididymis.  The  remaining 
mesonephrotic  tubules,  those  of  the  posterior,  or  exclusively 
urinary  portion  in  lower  forms,  never  develop  into  functional 
organs,  but  one  or  two  of  them,  with  blind  free  ends,  may 
retain  their  connection  with  the  Wolffian  duct,  and  form  the 
so-called  vasa  aberrantia,  while  the  remainder,  without  con- 
nection at  either  end,  form  a  rudiment  termed  the  paradidymis 
("Organ  of  Giraldes"),  the  homologue  of  the  paroophoron, 
of  the  female.  The  Wolffian  duct,  freed  from  all  association 
from  urinary  functions,  becomes  the  exclusive  spermatic  duct, 
the  ductus  deferens  (vas  deferens).  The  Miillerian  duct  is 
lost  along  the  greater  portion  of  its  extent,  but  leaves  rudi- 
ments at  either  end.  The  anterior  end  is  represented  by  the 
appendix  testis  [hydatid  of  Morgagni],  a  knobbed  body  at- 


THE   URO-GENITAL    SYSTEM  393 

tached  to  the  epididymis;  the  posterior  by  a  median  vesicle 
which  leads  from  the  dorsal  wall  of  the  urethra  and  lies  im- 
bedded in  the  prostate  gland,  the  prostatic  vesicle,  sometimes 
referred  to  as  the  uterus  masculinus. 

The  history  of  the  urogenital  organs,  as  thus  far  consid- 
ered, with  their  correspondence  in  the  two  sexes,  may  be  con- 
veniently shown  in  a  table,  in  which  the  first  column  gives  the 
part  in  its  primary  morphological  significance,  while  the  second 
and  third  state  their  ultimate  fate  in  the  male  and  female 
Amniota  respectively.  Vestigeal  parts  are  given  in  italics. 
This  table  may  be  studied  in  connection  with  the  one  at  the 
end  of  the  chapter,  in  which  the  external  parts  are  considered 
in  the  same  way. 

In  the  monotremes  the  ductus  deferentes  [vasa  deferentia] 
open  into  a  urogenital  sinus,  the  ventral  recess  of  a  common 
cloaca,  in  common  with  the  urethra  or  excurrent  duct  of  the 
urinary  bladder;  in  all  higher  mammals,  however,  with  the 
formation  of  a  perinaeum  or  division  between  this  uro-genital 
sinus  and  the  rectum,  the  ductus  deferentes  are  received  by 
the  much  prolonged  urethra  so  that  the  distal  portion  of  this 
is  a  common  duct  for -both  urinary  and  reproductive  products, 
a  resumption  of  early  conditions  under  another  form. 

While  in  the  monotremes  and  in  certain  placental  mammals 
the  testes  remain  throughout  life  in  or  near  the  original  posi- 
tion, in  others  they  experience  a  more  or  less  marked  change 
of  location.  This  is  termed  the  descensus  testiculorum,  and 
is  more  or  less  comparable  to  a  similar  descent  on  the  part  of 
the  ovaries,  although  the  procedure  involves  different  parts, 
and  is  quite  likely  of  a  different  historical  significance.  Aside 
from  the  monotremes,  no  appreciable  descent  takes  place  in 
elephants  and  in  certain  insectivores,  while  in  sloths  and  ant- 
eaters  the  testes  descend  considerably  and  take  up  a  final  posi- 
tion in  peritoneal  folds  between  bladder  and  rectum,  but  still 
within  the  pelvic  cavity.  The  only  other  placental  mammals 
in  which  there  is  no  external  manifestation  of  this  process  are 
the  armadillos,  related  to  these  last,  and  the  two  aquatic  or- 
ders of  Cetacea  and  Sirenia,  in  which  the  condition  is  plainly  a 


394 


HISTORY   OF   THE    HUMAN    BODY 


secondary  modification  due  to  the  needs  of  an  aquatic  life.  In 
all  remaining  mammals  the  process  is  connected  with  the  for- 
mation of  an  inguinal  canal,  a  subcutaneous  evagination  of 
the  body  wall  involving  muscles  and  peritoneum,  and  the 
testes  pass  into  this  either  periodically,  in  association  with 
sexual  activity,  or  permanently.  The  former  condition  occurs 


MORPHOLOGICAL  DESIGNATION 
(Embryonic  or  phylogenetic) 

MALE 
AMNIOTE 

FEMALE  AMNIOTE 

Germ  gland 

Testis 

Ovary 

Mesonephros  (upper  portion) 

Epididymis 

Epo'dphoron  (in  part) 

Ductuli 

Mesonephros  (lower  portion) 

aberrantes 
Paradidymis 

Paro'dphoron 

Mesonephrotic  (Wolffian)  duct. 

Ductus  de- 
ferens 

Epo'dphoron  (in  part) 
Longitudinal  duct  of 
epoophoron 

Mailer's  duct 

Appendix 
testis 
Vesicula 
prostatica 

Oviduct 
Uterus 
Vagina 

Urogenital  sinus 

Morphologi- 
cal urethra, 
/'.  e.  the  por- 
t  i  on    be- 

Urethra (entire) 

tween  the 

bladder  and 

entrance  of 

the  ductus 

deferentia 

among  many  insectivores  and  rodents,  and  in  the  bats;  the 
latter  is  characteristic  of  the  land  carnivora,  ungulates,  most 
lemurs  and  the  primates. 

In  the  majority  of  animals  coming  under  this  latter  head, 
that  of  permanent  descent,  the  testes  lie  in  a  special  integumen- 
tal  sac,  the  scrotum,  but  in  some  cases,  as  tapirs,  rhinoceros, 
etc.,  there  is  no  definite  scrotum,  and  the  testes  lie  beneath  the 


THE   URO-GENITAL    SYSTEM  395 

integument  of  either  the  inguinal  or  perinaeal  regions.  The 
scrotum  is  originally  double,  furnishing  a  separate  sac  for  each 
testis,  but  usually  the  two  are  fused  into  single  median  sac  in 
which  the  suture  of  union  is  usually  apparent.  In  relation  to 
the  penis  the  scrotum  is  originally  anterior  to  it,  prepenial,  as 
in  all  marsupials  that  possess  one,  but  in  placental  mammals 
these  relations  are  reversed  and  the  scrotum  becomes  postfcnial 
through  the  migration  of  the  penis  in  an  anterior  direction. 
No  definite  cause  for  the  descensus  is  known,  either  phylo- 
genetic  or  physiological,  and  the  phenomenon  has  gained  rather 
than  lost  in  complexity  through  recent  researches  which  show 
the  cooperation  of  several  distinct  elements  previously  not 
taken  into  consideration.  Formerly  a  mechanical  explanation 
was  found  in  the  gradual  contraction  of  the  band  of  perito- 
neum which  extends  from  the  testis  to  the  inguinal  region 
(Plate  III,  b),  and  termed  the  gubernaculum  in  reference  to 
its  supposed  function,  but  the  matter  is  not  as  simple  as  that, 
since  this  band  itself  is  composed  of  several  originally  dis- 
tinct elements,  and,  furthermore,  can  hardly  be  considered  to 
exert  the  tension  ascribed  to  it.  The  initiative  in  the  process 
seems  to  be  a  slight  invagination  of  the  abdominal  wall  at  the 
point  of  insertion  of  the  inguinal  ligament.  Through  a  sub- 
sequent evagination  followed  by  a  second  invagination  a  coni- 
cal body  is  formed,  the  conus  inguinalis  (Fig.  in,  A),  which 
involves  the  muscular  layers,  and  by  a  final  outpushing  of 
this  and  the  surrounding  structures  a  subcutaneous  muscular 
pouch  is  formed,  the  bursa  inguinalis,  in  the  bottom  of  which 
lies  the  conus,  which  serves  as  a  point  of  insertion  of  the  in- 
guinal ligament.  The  bursa  is  lined  by  a  pocket  of  perito- 
neum, the  processus  vaginalis,  which  is  reflected  up  over  the 
conus.  The  inward  development  of  the  conus  absorbs  and 
shortens  the  inguinal  ligament,  and  eventually  the  testis  comes 
to  lie  in  the  bursa,  covered  internally  by  the  reflected  peri- 
toneum. As  shown  above,  the  bursa  may  or  may  not  become 
placed  in  a  scrotal  sac,  but  when  it  does,  a  scrotal  ligament 
(chorda  gubernaculi)  extends  from  the  bottom  of  this  sac  to 
the  base  of  the  conus. 


396  HISTORY   OF    THE    HUMAN    BODY 

In  a  complete,  or  typical  descensus,  in  which  the  bursa  is 
contained  in  a  scrotal  sac,  the  parts  are  related  as  in  Fig.  in, 
B.  The  processus  vaginalis  of  the  peritoneum,  continuous 
beyond  the  sac  with  that  which  lines  the  abdominal  wall, 
wraps  itself  partly  around  testis  and  epididymis,  thus  forming 
a  membrane,  the  tunica  vaginalis  propria,  with  a  parietal  and 
visceral  layer,  and  a  serous  cavity  included  between  them. 
This  serous  cavity  is  naturally  continuous  with  the  main  ab- 
dominal cavity,  the  ccelom,  and  the  passage  between  them  re- 
mains open  in  those  mammals  in  which  the  external  appearance 
of  the  testes  is  periodic;  in  those,  however,  in  which  the 
descent  is  final  and  definite,  it  closes  up  during  late,  often 
post-natal,  development,  and  all  communication  between  the 
two  cavities  is  lost.  The  vessels  and  nerves  of  the  testis,  to 
which  is  added  the  ductus  deferens,  become  united  by  con- 
nective tissue  into  a  single  structure,  the  spermatic  cord,  which 
escapes  from  the  testis  along  the  side  not  invested  by  perito- 
neum, becomes  recurved  and  enters  the  abdominal  cavity  by 
running  along  the  wall  of  the  pouch,  covered  by  the  parietal 
layers  of  peritoneum  i.e.,  the  tunica  vaginalis  propria. 

Outside  of  this  come  three  layers  which  represent  the  ab- 
dominal muscles  and  their  fascia;  in  order,  beginning  from 
within:  i,  the  tunica  vaginalis  communis,  i.e.,  common  to  both 
testis  and  spermatic  cord,  a  continuation  of  the  fascia  trans- 
versa ;  2,  the  cremaster  muscle,  a  continuation  of  the  trans- 
versalis  and  internal  oblique,  and  3,  the  fascia  cremast  erica 
[Cooper's],  which  represents  the  external  oblique,  but  is  with- 
out muscular  fibers. 

Beyond  this  comes  the  integument,  although  this  is  often 
differentiated  into  two  layers  through  the  development  of  its 
involuntary  muscular  fibers  into  a  layer  of  integumental  mus- 
cles, the  tunica  dartos,  which  occasions  a  wrinkling  of  the 
surface  in  response  to  slight  stimuli. 

The  external  reproductive  organs  have  arisen  as  one  of  the 
adaptations  required  by  the  assumption  of  a  terrestrial  exist- 
ence, the  ultimate  cause  being  found  in  the  non-suitability  of 
the  air  as  a  medium  for  the  transmission  of  the  spermatozoa. 


THE   URO-GENITAL    SYSTEM 


397 


These  delicate  motile  cells  can  exist  only  in  a  liquid  medium, 
and  from  this  cause  alone  arise  in  all  terrestrial  animals  the 
necessities,  first,  of  secreting  a  liquid  to  serve  as  a  vehicle  for 
the  male  germ  cells,  and  second,  of  developing  organs  through 
which  this  liquid  may  be  directly  transmitted  to  the  cavities 
of  the  female  organs,  without  suffering  from  the  drying  action 
of  the  air.  That  this  necessity  was  not  immediately  apparent 


FIG.  in.  Diagrams  illustrating  the  descent  of  the  testes  in  mammals. 
[After  WEBER.] 

t,  testis;  p,  epididymis;  s,  spermatic  cord;  m,  mesorchium;  It,  ligamentum  testis; 
/*,  ligamentum  inguinale;  i,  bursa  inguinalis;  x,  conus  inguinalis;  y,  chorda  guber- 
naculi  (  =  ligamentum  scroti)  ;  a,  tunica  vaginalis  propria,  visceral  layer;  b,  the  same, 
parietal  layer;  c,  tunica  vaginalis  communis,  continuous  with  the  fascia  trans- 
versa;  d,  cremaster  (=transversalis-obliq.  int.  abdom.);  e,  fascia  cremasterica  Cooped 
(  — obliq.  ext.  abdom.);  f,  integument,  including  the  tunica  dartos  and  involuntary 
muscular  layer. 

is  due  to  the  semi-aquatic  habits  of  most  amphibians,  even  the 
most  terrestrial  of  which  resort  to  the  water  at  the  breeding 
season  and  are  thus  able  to  dispense  with  any  external  mech- 
anism; yet  here,  notwithstanding  the  absence  of  external 
organs,  there  have  arisen  numerous  habits,  such  as  the  love 
antics  of  salamanders  and  the  amplexation  of  frogs  and  toads, 
which  are  designed  to  secure  a  greater  likelihood  of  fertiliza- 


398  HISTORY   OF   THE    HUMAN    BODY 

tion  and  thus  form  the  prelude  to  the  development  of  a  gen- 
uine copulation. 

It  is  evident,  however,  that  with  the  complete  relinquish- 
ment  of  an  aquatic  life,  and  the  subsequent  impossibility  of 
employing  an  external  vehicle  for  the  conveyance  of  the  sper- 
matozoa, some  method  must  be  found  by  means  of  which  the 
seminal  fluid  may  be  conveyed  direct  from  the  male  to  the 
female;  and  this  process,  beginning  with  the  most  natural 
stage  of  the  approximation  of  the  two  unmodified  cloacae, 
would  develop  first  a  temporary  evagination  of  a  portion  of  the 
inner  cloacal  wall,  and  then  a  permanent  modification  of  this 
evaginating  portion;  a  development  which  would  naturally 
take  place  in  the  male  alone,  as  the  producer  of  the  fluid  to  be 
transferred.  There  thus  arises  for  the  first  time  in  vertebrates 
an  intromittent  organ  or  penis,  three  distinct  types  of  which 
are  found;  these  appear  to  have  arisen  independently,  al- 
though in  all  cases  by  a  modification  of  the  cloacal  wall.  The 
first  is  seen  in  those  highly  specialized  burrowing  amphibians, 
the  Gymnophiona,  and  consists  of  a  protrusible  tube  worked 
by  muscles ;  the  second  is  that  of  lizards  and  snakes,  and  is  in 
the  form  of  two  lateral  protrusible  sacs,  the  walls  of  which  are 
often  cornified,  and  possess  a  spiral  groove  for  the  convey- 
ance of  the  spermatic  fluid ;  the  third  occurs  in  its  simplest 
form  in  turtles  and  crocodiles  and  suggests  a  terrestrial  origin 
for  both  groups.  This  latter  is  the  type  from  which  the  penis 
of  both  birds  and  mammals  is  derived,  and  may  be  described 
more  at  length.  Owing  to  the  imperfectly  understood  law  of 
sexual  homology  which  obtains  among  vertebrates,  this  organ, 
sometimes  termed  the  phallus  to  distinguish  it  from  the  other 
types,  exists  also  in  the  female  in  a  much  reduced  form,  and 
is  termed  the  clitoris.  Although  useless  as  an  intromittent 
organ,  it  reflects  the  peculiarities  of  the  male  organ  and  in  the 
various  groups  often  shows  in  a  reduced  form  the  characteris- 
tics developed  by  the  latter. 

The  phallus  develops  from  the  ventral  wall  of  the  cloaca 
and  consists  of  a  longitudinal  thickening  of  fibrous  tissue,  the 
corpus  fibrosum,  upon  which  rests  a  mass  of  cavernous  (erec- 


THE    URO-GENITAL    SYSTEM  399 

tile)  tissue  in  the  form  of  two  lateral  ridges,  the  corpora 
cavernosa,  with  a  median  groove  between  them,  the  seminal 
groove.  This  entire  organ  is  somewhat  tongue-shaped  and 
free  at  the  tip,  and  is  capable  of  considerably  protrusion  be- 
yond the  cloacal  orifice.  The  urogenital  sinus,  bearing  the 
openings  of  the  ductus  deferentes,  opens  into  the  seminal 
groove  near  its  proximal  end. 

Although  the  phallus  of  these  reptilian  forms  seems  at  first 
sight  quite  distinct  from  that  of  mammals,  and  although  there 
exist  at  present  no  transition  forms  among  adult  animals, 
the  development  of  these  parts  in  mammals  supplies  the  missing 
portions  of  the  history  and  substantiates  the  homology.  The 
essential  change  is  that  of  the  conversion  of  the  spermatic 
groove  into  a  complete  tube,  which  is  accomplished  by  the 
increase  in  size  of  the  lateral  ridges  and  their  subsequent 
fusion,  a  process  repeated  during  early  development.  The 
failure  to  complete  this  produces  the  condition  known  as 
hypospadias,  and  is  thus  seen  to  be  a  case  of  arrested  develop- 
ment, the  retention  of  the  reptilian  stage. 

The  relative  position  of  the  penis  changes  completely  during 
its  mammalian  history  from  a  post-scrotal  one  with  the  free 
end  directed  posteriorly  to  one  that  is  pre-scrotal  and  directed 
anteriorly.  The  first  of  these  positions  is  similar  to  that  of  the 
turtles  and  crocodiles  and  is  seen  in  the  monotremes,  and  to 
a  lesser  extent  in  marsupials ;  the  latter  position  is  characteris- 
tic of  placental  mammals.  This  change  may  be  made  clear 
by  the  accompanying  diagrams  (Fig.  112). 

In  the  monotremes  the  conditions  are  still  essentially  rep- 
tilian. There  is  a  common  cloaca  and  the  penis  projects  a 
little  from  its  ventral  wall.  The  ureters,  ductus  deferentes  and 
urinary  bladder  form  a  common  duct  which  under  normal  con- 
ditions serves  merely  as  a  passage  for  the  urine.  This  duct 
is  morphologically  the  urethra  as  far  as  the  entrance  of  the 
ductus  deferentes  and  ureters ;  beyond  this  point  it  is  morpho- 
logically the  urogenital  sinus.  The  erection  of  the  penis, 
through  the  slight  lengthening  of  its  inner  end,  closes  the 
entrance  into  the  cloaca,  but  continues  the  urogenital  canal 


400 


HISTORY   OF   THE    HUMAN    BODY 


into  its  own  lumen,  thus  forming  a  direct  outlet  from  the 
ductus  deferentes  to  the  exterior.  At  the  same  time  the  free 
end  becomes  protruded  from  the  cloacal  orifice,  and  the  organs, 


a 


FIG.  112.  Relationships  of  the  male  urogenital  organs  in  mammals. 
[After  WEBER.] 

(a)    Monotremes.      (b)    Marsupials.      (c)    Placental    mammals. 

s,  symphysis  pubis;  in,  intestine;  v,  urinary  bladder;  u,  ureter;  t,  testis;  w,  ductus 
deferens;  p,  prostate;  g,  vesicular  gland;  c,  bulbo-urethral  gland;  cp,  corpus 
cavernosum  penis;  cu,  corpus  cavernosum  urethrze  (^corpus  spongiosum). 


THE   URO-GENITAL    SYSTEM  401 

usually  wholly  subservient  to  the  urinary  function,  become 
for  the  time  being  wholly  reproductive.  % 

The  marsupials  show  an  intermediate  condition  by  which 
the  transition  to  the  placental  mammals  can  be  explained.  The 
cloaca  has  been  divided  by  a  perinaeum  and  the  alimentary  and 
urogenital  outlets  have  become  entirely  separated.  The  testes 
show  a  marked  descensus  and  usually  come  to  lie  in  a  scrotal 
sac,  which  is  prepenial  in  position.  The  penis  is  posterior  to 
the  testes  and  is  still  directed  backwards  as  in  monotremes  and 
sauropsids,  but  becomes  attached  at  its  proximal  end  to  the 
posterior  margin  of  the  os  pubis. 

The  true  urethra  is  very  short,  as  the  ductus  deferentes  enter 
the  tube  soon  after  its  origin,  but  the  urogenital  tube  thus 
formed  is  permanently  continuous  with  the  lumen  of  the  penis, 
forming  a  long  urogenital  canal.  This  condition  is  essentially 
that  found  in  placental  mammals  except  for  the  relative  posi- 
tion of  the  penis,  which  in  the  latter  animals,  retaining  its 
proximal  attachment  to  the  lower  margin  of  the  os  pubis,  turns 
about  and  becomes  directed  anteriorly,  thus  changing  its  ap- 
parent relations  with  the  testes,  which  are  now  post-penial. 

Connected  with  the  penis  are  various  sorts  of  glands,  em- 
ployed mainly  for  the  purpose  of  furnishing  a  liquid  vehicle 
for  the  spermatozoa.  They  are  thus  the  most  widely  developed 
in  mammals  of  marked  fertility,  like  rodents  and  insectivores, 
and  may  be  arranged  in  five  groups,  each  associated  with  a 
definitive  part  of  the  spermatic  tract  (Fig.  113).  The 
glandules  ductus  deferentis  are  thickenings  of  the  wall  of  the 
ductus  deferens,  and  are  situated  near  its  entrance  into  the 
urogenital  canal.  The  glandule?  vesicates  are  large  and  evi- 
dent glands,  which  open  near  the  latter.  These  have  often 
been  considered  as  receptacles  for  the  spermatic  fluid,  and  are 
hence  usually  called  seminal  vesicles,  but  they  are  clearly  glan- 
dular in  their  nature,  and  their  cavities  contain  spermatozoa 
only  by  accident.  Of  the  remaining  three,  which  open  into 
the  urogenital  canal,  the  primitive  condition  is  seen  in  the 
urethral  glands,  tubular  glands  occurring  in  the  walls  of 
the  above  canal,  especially  along  its  proximal  portion.  From 


402 


HISTORY   OF  THE   HUMAN   BODY 


such  elementary  structures  are  derived  the  two  other  sets,  the 
prostate  and  the  bulbo-urethral  [Cowper's].  Of  these  the 
former  are  more  proximal  in  position,  the  latter  more  distal. 
The  function  of  these  five  sets  of  glands  seems  in  all  cases 
that  given  above,  and  their  occurrence  in  the  various  mam- 


FIG.  113.    Penis  of  placental  mammals. 

(A)  Mouse  (.Mus  musculus).  [Combined  from  RAUTHER  and  OPPEL.]  (B) 
Hedgehog  (.Erinaceus  curopaeus.)  [From  OPPEL,  after  SEUBERT.] 

k,  kidney;  u,  ureter;  b,  bladder;  t,  testis;  e,  epididymis;  v.  d.,  ductus  deferens; 
cc,  corpora  cavernosa;  v,  vesicular  glands;  pr,  prostate  glands;  c,  bulbo-urethral 
(Cowper's)  glands;  p,  preputial  glands. 

mals  is  such  that  the  large  development  of  one  is  compensatory 
for  the  small  size  of  another. 

Thus  in  monotremes  and  marsupials  there  is  no  prostate 
gland,  but  the  urethral  glands  are  very  abundant ;  the  vesicular 
glands  are  wanting  in  carnivores,  but  large  and  well  developed 
in  primates.  In  Man  the  most  important  of  these  is  the  pros- 
tate, but  the  vesicular  are  also  well  developed.  The  bulbo- 
urethral  glands  are  evident  but  not  voluminous.  In  addition 


THE   URO-GENITAL    SYSTEM  403 

to  the  above  glands,  the  function  of  which  is  to  furnish  a 
liquid  vehicle  for  the  spermatozoa,  occur  certain  modified  in- 
tegumental  glands,  like  the  preputial,  the  function  of  which 
is  to  lubricate  the  parts. 

The  external  organs  of  the  female  are  but  slightly  developed 
and  appear  to  represent  the  various  elements  found  in  the 
male,  though  retained  permanently  in  a  reduced  and  almost 
embryonic  condition.  This  is  best  shown  by  a  comparison  of 
the  two  as  they  appear  in  development,  differentiating  from 
an  indifferent  condition  common  to  both,  as  in  the  case  of  the 
internal  parts.  As  this  history  begins  with  a  simple  cloaca 
and  develops  the  external  parts  from  its  walls  and  margin,  the 
history  recapitulates  also,  in  a  very  complete  fashion,  the 
stages  shown  phylogenetically  in  the  preceding  pages  (Fig. 
114). 

In  an  early  human  embryo  the  cloacal  orifice  is  approxi- 
mately circular  in  shape  and  is  surrounded  by  a  rounded  and 
somewhat  elevated  margin,  the  genital  ridge.  From  within 
its  ventral  wall,  and  projecting  a  little  beyond  the  cloacal  ori- 
fice, rises  a  conical  papilla,  the  genital  tubercle  [g],  which  is 
really  in  the  form  of  an  inverted  trough,  enclosing  the  uro- 
genital  sinus  and  freely  open  along  its  ventral  aspect,  thus 
forming  the  genital  cleft  [r].  At  a  later  stage  the  cloacal 
orifice  becomes  more  prolonged  dorso-ventrally,  and  the  genital 
ridge  has  become  more  pronounced  along  the  edges,  forming 
two  lateral  ridges  [h~\,  instead  of  a  circular  lip.  The  genital 
tubercle  has  also  developed  and  projects  conspicuously  from 
the  ventral  margin  of  the  orifice;  its  groove  is  still  conspicu- 
ous, but  not  so  widely  open,  and  its  lateral  lips  take  on  the 
aspect  of  rounded  folds  [c~\ .  The  terminal  end  of  the  rectum 
has  become  visible  and  forms  an  anus,  distinct  from  the  gen- 
ital parts,  but  almost  continuous  with  them. 

Thus  far  the  conditions  in  the  two  sexes  are  precisely  alike 
and  the  stages  are  termed  indifferent,  although  we  have  reason 
to  believe  that  the  sex  determination  is  made  at  a  far  earlier 
period  than  the  -first  one  considered  here,  probably  even  in  the 
fertilised  egg  previous  to  segmentation. 


404 


HISTORY   OF   THE    HUMAN    BODY 


At  about  this  point,  however,  sexual  differences  begin  to 
appear,  as  may  be  seen  by  a  comparison  of  the  remaining 
figures.  The  female  organs,  which  remain  nearer  the  embry- 
onal condition,  are  not  essentially  different,  save  in  propor- 
tions, from  the  last  stage  common  to  both.  The  genital  cleft 
remains  open,  forming  the  introitus  vagince,  into  which  empty 
the  united  Miillerian  ducts  (uterus)  and  the  two  ureters.  The 
genital  folds  form  the  corpora  cavernosa  (labia  minor  a  or 


a 


h-\— 


FIG.  114.    Development  of  the  external  genitals  in  Man. 

(a)  and  (b)  Indifferent  stages,  (c)  Early  stage  of  the  male  organs,  (d)  Early 
stage  of  the  female  organs. 

g,  genital  turbercle;  c,  genital  folds;  h,  genital  ridges;  r,  genital  cleft;  ra,  anus; 
pt  perinaeum. 

nympha)  and  the  free  tubercle  itself  forms  the  clitoris.  The 
external  lips  of  the  cloaca,  the  lateral  genital  ridges,  form  the 
greater  labia  (labia  major  a). 

In  the  male  the  genital  tubercle  develops  into  the  glans  and 
corpus  cavernosum  urethra  [corpus  spongiosum],  and  the 
genital  folds  become  the  corpora  cavernosa  penis.  By  the 
fusion  of  these  latter  the  groove  becomes  converted  into  the 
uro-genital  canal,  which  becomes  continuous  with  the  urethra, 


THE   URO-GENITAL    SYSTEM 


405 


and  into  which  the  ductus  deferentes  empty.  The  lateral  gen- 
ital ridges  unite  to  form  the  scrota!  sac,  and  the  point  of  union 
between  these  is  marked  by  a  raphc.  A  median  line  of  dark 
pigment  lies  along  the  under  side  .of  the  penis  continuous  with 
the  latter  and  marks  the  fusion  of  the  lips  of  the  genital 
groove.  Special  muscles,  also  in  part  sexually  homologous, 
develop  in  connection  with  the  external  organs  of  both  sexes. 
These  are  composed  of  striated  fibers  and  are  more  or  less 
under  the  control  of  the  will. 

The  elements  of  the  indifferent  stage  and  their  differentia- 
tions in  the  two  sexes  may  be  expressed  in  the  following  table, 
which  shows  the  sexual  homologies.  This  table,  taken  in 
connection  with  the  one  given  above,  for  the  internal  parts,  will 
form  a  brief  synopsis  of  the  entire  subject. 


EMBRYONAL  PART 

MALE 

FEMALE 

Genital  ridges 

Scrotum 

Labia  majora 

Genital  tubercle 

Corpus  c'avernosum 
urethrae 
Glans  penis 

Clitoris 

Genital  cleft 

Pigmented  line 

Introitus  vaginae 

Genital  folds 

Corpora  cavernosa 
penis 

Labia  minora 

CHAPTER    X 
THE  NERVOUS  SYSTEM 

"  Indeed,  while  Nature  is  wonderfully  inventive  of  new 
structures,  her  conservatism  in  holding  on  to  old  ones 
is  still  more  remarkable.  In  the  ascending  line  of  de- 
velopment she  tries  an  experiment  once  exceedingly 
thorough,  and  then  the  question  is  solved  for  all  time. 
For  she  always  takes  time  enough  to  try  the  experi- 
ment exhaustively.  It  took  ages  to  find  how  to  build 
a  spinal  column  or  brain,  but  when  the  experiment  was 
finished  she  had  reason  to  be,  and- was,  satisfied." 

JOHN   TYLER,   The    Whence   and   Whither   of 

Man,  p.  173. 

THE  central  nervous  system  begins  its  history  as  a  straight 
tube  lying  along  the  mid-dorsal  line  just  beneath  the  integ- 
ument. Anteriorly  this  tube  ends  blindly  and  exhibits  a  series 
of  three  vesicular  enlargements,  the  beginnings  of  the  brain ; 
posteriorly  it  ends  blindly  also  and  tapers  to  a  point,  although 
there  are  certain  mysterious  indications  in  the  embryonic 
record  of  a  former  connection  with  the  lumen  of  the  alimentary 
canal,  indications  which  have  not  as  yet  received  any  satis- 
factory explanation,  and  which  may  be  after  all  merely  de- 
velopmental necessities,  without  historic  significance.  Through 
modification  of  this  simple  neural  tube  without  the  addition  of 
extraneous  elements  save  as  auxiliary  to  this,  there  arise  in  all 
vertebrates  the  brain  and  spinal  cord,  which,  even  in  their 
highest  and  most  complicated  form,  appear  to  the  morpholo- 
gist  as  still  tubular;  the  walls,  enormously  thickened  in  places 
and  often  folded,  give  rise  to  such  solid  masses  as  the  cere- 
bellum or  the  cerebral  hemispheres,  the  lumen  persists  as  the 
ventricles  of  the  brain,  and  their  continuation  through  the 
spinal  cord  as  the  canalis  centralis. 

All  nervous  systems  have  arisen  in  the  beginning  in  response 
to  stimuli  from  without,  and  hence  developed  originally  upon 

406 


THE    NERVOUS    SYSTEM 


the  surface  of  the  body,  especially  upon  that  /portion  whichjr 
through  the  customary  position  of  the  body,  isjfl^€xposed  to 
such  stimuli.  Such  superficial  systems  are  still  found  among 
lower  invertebrates ;  in  sessile  radiate  forms  equally  developed 
on  all  sides  of  the  projecting  rim  or  upon  the  tentacles,  in  free- 
swimming  forms  as  an  apical  plate  located  upon  the  point 
which  first  comes  in  contact  with  external  objects.  That  such 
was  also  the  case  with  the  unknown  ancestor  of  vertebrates 
is  suggested  by  the  embryonic  history  of  the  neural  tube,  for 
it  i.y  formed  here  by  the  rolling  in  of  the  external  dorsal  sur- 
face of  the  early  embryo.  This  process  is  inaugurated  by  the 
formation  of  two  longitudinal  medullary  folds,  one  upon  either 
side  of  the  middle  line,  and  as  these  are  united  around  the 
anterior  end  and  diverge  posteriorly,  they  form  for  a  time  a 
figure  not  unlike  that  of  an  ordinary  hair-pin.  The  area  en- 
closed by  these,  which  consists  of  a  strip  along  the  dorsal 
surface,  becomes  somewhat  sunken,  and  as  the  two  medullary 
folds,  beginning  anteriorly,  approach  one  another,  and  finally 
unite,  the  area  becomes  the  bottom  of  a  trough,  and  eventually 
the  inner  surface  of  a  tube. 

The  complete  coalescence  of  the  folds  and  the  pinching  off 
of  the  trough  are  the  final  steps  in  the  process,  which  results 
in  the  formation  of  the  neural  tube  as  described  above,  the 
anlage  of  the  central  nervous  system.  If  we  may  take  this 
process  as  a  recapitulation  of  pre-vertebrate  conditions,  a  view 
sustained  by  its  universality  and  the  reasonableness  of  the  con- 
clusions, it  suggests  that  the  primitive  ancestor  of  vertebrates 
was  exposed  to  external  stimuli  mainly  over  its  dorsal  surface, 
a  supposition  which  in  its  turn  suggests  a  slightly  flattened, 
worm-like  form,  with  the  ventral  side  resting  upon  the  ground, 
here  undoubtedly  the  ocean  bottom.  The  greater  development 
of  the  anterior  portion  of  this  tube,  even  from  the  first,  sug- 
gests a  locomotive  habit,  which  would  thus  favor  the  anterior 
end  in  this  regard.  As  this  superficial  nervous  system  became 
more  highly  developed,  and  hence  more  sensitive,  it  was  pro- 
tected in  the  most  natural  way  for  such  a  system,  by  the  for- 
mation of  elevated  ridges  along  its  lateral  borders,  thus  form- 


408  HISTORY   OF   THE    HUMAN    BODY 

ing  a  dell  or  trough,  in  the  bottom  of  which  lay  the  sensitive 
surface.  Such  a  method  of  protection,  once  inaugurated,  could 
have  but  one  logical  outcome,  the  gradual  formation  of  a  tube 
through  the  increase  in  height  and  the  approximation  of  the 
protecting  lateral  folds,  until  in  this  way  the  form  was  at- 
tained with  which  all  the  present-day  vertebrates  are  equipped. 
We  must  here  not  lose  sight  of  the  fact  that  the  original  ex- 
ternal, and  hence  the  sensitive,  surface  is  not  that  of  the  ex- 
terior, but  that  of  the  lumen  of  the  tube,  which  explains  the 
fact,  to  be  developed  later,  that  in  all  lower  vertebrates  the 
central  or  ganglion  cells,  which  form  the  "  gray  matter,"  are 
situated  along  the  lumen,  and  not  along  the  external  surface, 
a  condition  retained  throughout  in  the  more  conservative 
spinal  cord,  although  secondarily  in  the  higher  forms  large 
masses  of  gray  matter  develop  also  over  the  external  surface 
of  parts  of  the  brain. 

A  central  nervous  system,  by  thus  sinking  into  the  interior 
and  becoming  entirely  covered  up  by  a  much  less  sensitive 
surface,  gains  the  protection  which  it  seeks,  but,  in  order  to 
retain  its  functicn  as  a  receiver  of  external  stimuli,  a  function 
upon  which  its  very  existence  as  a  nervous  system  depends, 
it  must  retain  its  connection  with  the  exterior  through  sets  of 
secondary  cells  which  remain  external  and  are  yet  continuous 
with  the  central  organ. 

These  are  the  sensory  cells,  which,  when  grouped  over  a 
certain  area  and  specialized  to  receive  a  certain  form  of 
stimulus,  become  definite  sense-organs.  These  are  connected 
with  the  central  system  by  sensory  nerves,  in  which  the  direc- 
tion of  the  impulse  is  always  from  without  inward,  that  is, 
afferent  or  centripetal.  As  the  sensory  cells  become  themselves 
more  specialized  and  hence  more  sensitive  as  well  as  more 
vital  to  the  organism,  they  themselves  need  protection,  which 
they  obtain  either  by  the  formation  of  an  external  non-sensi- 
tive horny  layer,  the  epidermis,  which  protects  the  sensory 
cells  scattered  over  the  general  surface  while  it  still  allows  the 
transmission  of  the  stimuli ;  or,  in  the  case  of  such  special 
sense  organs  as  the  patches  of  sensory  cells  that  form  the  es- 


THE    NERVOUS    SYSTEM  409 

sential  organs  of  vision  and  hearing-  (retina  and  acoustic  mac- 
ula) elaborate  series  of  protective  organs  become  developed, 
while  at  the  same  time  the  special  stimuli  are  intensified  by 
various  accessory  organs. 

Thus,  while  this  secondary  system  of  sensory  cells,  like 
the  rank  and  file  of  a  modern  army,  meets  the  external  world 
with  its  hazards,  the  central  nervous  system,  and  more  espe- 
cially the  brain,  like  the  general  staff,  remains  in  safety, 
though  in  constant  communication  with  the  front.  The  eyes 
see,  the  ears  hear,  the  outer  surface  receives  constant  evidence 
of  the  external  world,  while  the  brain,  immured  within  a 
dense  wall  of  bone,  sits  in  utter  darkness  and  silence.  It 
neither  hears  nor  sees;  no  ray  of  light  ever  penetrates  its 
obscurity,  and  even  when  exposed  through  injury  or  operation 
it  is  found  to  have  no  power  of  direct  perception  or  even  of 
sensation ;  and  yet  it  directs  the  entire  mechanism  with  the 
utmost  intelligence,  sending  its  messages  to  the  motor  system, 
and  causing  the  entire  body  to  act  in  the  strictest  harmony  with 
the  external  conditions.  In  the  performance  of  this  function 
it  has  developed  a  complexity  immeasurably  in  excess  of  that 
of  any  other  organ,  and  even  far  beyond  that  of  its  own  sense- 
organs,  since  these  latter  attain  a  high  degree  of  development 
among  fishes,  while  the  brain  continues  its  development 
through  amphibians  and  reptiles,  becomes  larger  and  more 
complex  among  the  mammals,  especially  along  the  line  leading 
to  the  anthropoids,  and  attains  its  highest  point  in  the  human 
species,  a  member  of  the  latter  Order,  not  otherwise  to  be 
especially  distinguished  from  the  remainder  of  the  group.  It 
is  thus  to  be  concluded  that  the  remarkable  development  of 
brain  characteristic  of  mammals  in  general,  and  the  Anthro- 
poidea  in  particular,  has  not  been  brought  about  through  a 
greater  perfection  of  the  sense-organs,  but  rather  by  increasing 
its  own  power  of  receiving  the  sensory  impressions  and  of 
recording  them  through  the  formation  of  association  paths; 
and  this,  like  all  other  structural  advances,  has  been  gradually 
brought  about  through  the  wrorking  of  natural  law,  as  a  more 
perfect  adaptation  to  environment. 


4io 


HISTORY   OF   THE    HUMAN    BODY 


The  material  history  of  this  advance  appears  to  the  mor- 
phologist  as  the  gradual  modification  of  the  simple  neural  tube 
described  above,  a  development  which  is  traceable  alike  in  the 
comparison  of  adult  animals,  Class  by  Class,  and  in  the  em- 
bryological  record  of  a  single  animal,  the  lower  forms  preserv- 
ing in  greater  detail  the  early  stages  of  the  history,  the  higher 


FIG.  115.     Diagrams  of  the  primary  and  secondary  cerebral  vesicles. 

(A)  The  primary  vesicles.  (B)  The  typical  form  of  brain  of  vertebrates  as  de- 
rived from  A. 

The  correspondence  between  the  two  is  indicated  by  the  horizontal  dotted  lines, 
•which  mark  off  the  areas  of  the  primary  vesicles,  I,  II,  and  III. 

forms  recording  the  later  stages.  Completed  in  this  way 
from  the  numberless  fragmentary  records  presented  to  the 
investigator,  the  history  of  the  neural  tube  in  its  progressive 
modifications  is  as  follows  : 

It  begins,  so  far  as  records  go,  with  a  form  in  which  the  an- 
terior part  of  the  tube,  that  corresponding  to  the  head  of  the 


THE   NERVOUS    SYSTEM  411 

animal,  is  somewhat  enlarged  and  divided  by  two  transverse 
constrictions  into  three  successive  vesicles  (Fig.  115,  A),  the 
fore-,  middle-,  and  hind-brain,  or,  more  technically,  prosen- 
cephalon,  mesencephalon  and  metencephalon,  each  with  its 
cavity  or  primary  ventricle.  Of  these  the  first  two  seem  to 
represent  the  very  rudimentary  cerebral  vesicle  found  in  Am- 
phioxus  and  may  be  termed  the  archencephalon,  or  primordial 
brain,  while  the  third  may  be  considered  the  anterior  end  of 
the  spinal  cord,  which  becomes  added  to  the  brain  at  some 
point  between  Amphioxus  and  the  cyclostomes.  This  cerebral 
vesicle  of  Amphioxus  bears  two  rudimentary  sense  organs,  an 
olfactory  groove  and  a  pigment  spot;  it  may  be  more  than  a 
coincidence,  then,  that  in  the  higher  forms  the  first  two  original 
vesicles  furnish  but  two  pairs  of  nerves,  olfactory  and  optic, 
while  the  other  nerves  are  derived  from  the  primary  third 
vesicle. 

Of  all  vertebrates  the  cyclostomes  alone  possess  a  brain 
which  may  be  interpreted  as  still  consisting  of  three  primary 
vesicles;  in  all  others  several  modifications  take  place  (Fig. 
115,  B).  The  prosencephalon  becomes  modified  by  the  forma- 
tion of  two  diverticula,  which  are  thrown  out  from  the  sides 
and  grow  anteriorly,  often  reaching  a  point  considerably  be- 
yond the  anterior  limits  of  the  primary  tube.  These  are  the 
two  lobes  of  the  telencephalon  (the  cerebral  hemispheres  of 
the  higher  vertebrates),  in  distinction  from  which  the  un- 
paired remainder  is  designated  as  the  diencephalon. 

Internally  the  primary  first  ventricle  becomes  divided  into 
the  two  lateral  ventricles  and  the  one  naturally  denominated 
the  third;  the  latter  communicates  with  the  two  first  through 
a  passage  which  is  inclined  to  become  narrow,  the  foramen  in- 
terventriculare  [foramen  of  Monro"]. 

The  mesencephalon  is  the  most  conservative  of  the  primary 
vesicles,  and  other  than  a  lateral  expansion  which  sometimes 
forms  a  pair  of  prominent  optic  lobes,  suffers  no  marked 
change.  Its  ventricle  is  often  large  and  obvious,  but  has  re- 
ceived no  special  name  or  number. 

The  third   primary  vesicle,   the  metencephalon,   shows   a 


412  HISTORY    OF    THE    HUMAN    BODY 

greater  or  less  differentiation  of  its  anterior  portion,  which 
forms  the  metencephalon  in  a  restricted  sense  (the  cerebellum 
of  higher  forms),  while  the  posterior  portion,  which  tapers  in- 
definitely into  the  spinal  cord,  is  distinguished  as  the  myelen- 
cephalon  or  medulla.*  The  ventricle  of  the  third  primary  vesi- 
cle, or  more  especially  that  of  the  myelencephalon,  is  a  large 
and  conspicuous  cavity  in  lower  vertebrates  and  in  the  embryo 
of  the  higher  ones,  and  is  known  as  the  fourth  ventricle.  That 
part  of  the  lumen  which  lies  between  this  and  the  third  ven- 
tricle, including  the  ventricle  of  the  mid-brain,  forms  in  Man 
a  small  tube  or  duct,  and  has  consequently  received  the  name 
of  aqueductus  cerebri  \_Sylvii],  or  the  "  iter  a  tertio  ad  quar- 
tum  ventriculurn." 

The  original  three  primary  cerebral  vesicles,  by  a  secondary 
subdivision  of  the  first  and  third,  thus  become  increased  to  five, 
and  form  a  fundamental  plan  to  which  the  brain  of  all  higher 
vertebrates  may  be  referred.  In  the  development  of  the  many 
forms  of  adult  brains  from  this  ground  plan  certain  mechanical 
principles  are  involved  which  it  is  well  to  consider  separately 
before  continuing  the  special  history.  These  mechanical 
principles  are  as  follows : 

i.  Increase  in  the  thickness  of  the  wall  over  a  definite  area. 

*The  ease  with  which  the  German  anatomists  have  translated  into  the 
vernacular  the  somewhat  ponderous  Greek  terms  for  the  parts  of  the 
brain  (Vorderhirn,  Zwischenhirn,  Mittelhirn,  etc.)  has  led  to  various 
attempts  on  the  part  of  English-speaking  scholars  to  emulate  their  ex- 
ample, but  with  varied  success.  Thus  "  fore-brain  "  and  "  mid-brain  "  for 
prosencephalon  and  mesencephalon  respectively  are  convenient  although 
somewhat  mediaeval,  and  these,  together  with  the  inelegant  "  hind- 
brain,"  are  now  in  general  use.  The  forms  "  after-brain  "  for  myelen- 
cephalon (Ger.  Nachhirn)  and  "  twixt-brain,"  or  "  tween-brain "  for 
diencephalon  (Ger.  Zwischenhirn)  are  less  happy,  and  it  is  doubtful 
if  they  will  ever  receive  general  favor.  The  Greek  terms  are,  on  the 
whole,  the  most  satisfactory,  and  are  more  in  accordance  with  our  usage 
than  are  their  rather  crude  Anglo-Saxon  equivalents. 

The  numbering  of  the  cerebral  ventricles  is  that  of  an  old  enumeration 
and  does,  not  at  all  correspond  with  the  morphological  value  of  the  parts. 
They  are  most  conveniently  named  in  accordance  with  the  vesicles  of  which 
they  form  the  cavities,  thus:  telocozles,  diaccele,  mesoccele,  metaccele  and 
myeloccele. 


THE    NERVOUS    SYSTEM  413 

This  is  seen  almost  everywhere,  but  the  extent  of  the  develop- 
ment in  thickness  varies  much.  It  is  well  shown  by  the  thick- 
ening of  the  floor  of  the  telencephalic  lobes,  forming  the 
corpora  striata,  or  by  that  of  the  roof  and  sides  of  the  same 
parts  in  the  higher  vertebrates,  forming  the  cerebral  hemi- 
spheres. 

2.  The  retention  of  the  embryonal  thinness  over  a  definite 
area,  forming  a  place  where  the  lumen  is  separated  from  the 
exterior  by  merely  a  thin,  often  a  transparent,  membrane.  Such 
places  are  extremely  puzzling,  and  misled  anatomists  until 
within  a  generation.  The  physiological  purpose  of  such  a  thin 
place  is  to  allow  the  blood  to  communicate  with  the  lymph  of 
the  ventricles  and  to  nourish  the  inner  surfaces  without  violat- 
ing the  integrity  of  the  original  neural  tube.  A  plexus  of 
blood-vessels  is  thus  the  constant  accompaniment  of  such  a 
thin  place,  and  the  relations  usually  become  still  more  compli- 
cated by  the  sinking  into  the  cavity  of  the  entire  structure, 
although  each  loop  of  capillaries  is  covered  and  veiled  by  the 
membranous  wall,  and  thus  the  integrity  of  the  tube  is  never 
violated.*  In  extreme  cases  almost  the  entire  thin  area,  cov- 
ering a  network  of  capillary  loops  and  following  its  intricacies, 
may  come  to  lie  within  the  cavity  of  a  ventricle  and  form  a  so- 
called  chorioid  plexus. 

The  most  important  of  these  organs  are  (i)  those  of  the 
lateral  ventricles,  formed  by  the  imagination  of  a  thin  area  in 
the  inner  wall  of  each,  (2)  a  similar  one  in  the  third  ventricle, 
invaginated  from  its  roof,  and  (3). one  formed  from  the  roof 
of  the  fourth  ventricle  immediately  behind  the  cerebellum. 

*  As  the  only  exception  to  this  rule  there  have  been  described  in  Man 
and  in  certain  other  mammals  one  or  more  small  perforations  in  the  roof 
of  ihe  fourth  ventricle,  the  foramina  of  Majendie,  which  form  a  direct 
communication  between  the  lumen  of  the  neural  tube  and  the  sub- 
arachnoid  space.  The  existence  of  this  communication,  which  violates 
the  morphological  principle  of  the  complete  integrity  of  the  walls  of  the 
neural  tube,  has  given  rise  to  much  discussion,  but  it  seems  now  probable 
that,  while  these  foramina  certainly  do  occur  occasionally,  it  is  an  individual 
peculiarity,  like  the  epitrochlear  foramen  of  the  humerus,  and  of  no 
especial  significance. 


414  HISTORY    OF    THE    HUMAN    BODY 

In  other  cases  localized  thin  areas  push  out  instead  of  in  and 
form  evaginations  of  more  or  less  importance  in  the  formation 
of  various  organs,  which  are  supplementary  to  the  actual  brain. 
In  this  way  are  formed  the  retina  of  the  eyes,  a  portion  of  the 
hypophysis,  and  one  or  two  problematic  structures  arising  dor- 
sally  from  the  diencephalon. 

3.  Folding  or  creasing  of  a  certain  part  of  the  wall.     This 
mechanical  device  means  here  as  elsewhere  an  increase  of  sur- 
face, and  hence  of  physiological  efficiency,  without  a  corre- 
sponding increase  in  bulk.     Its  best  manifestation  is,  perhaps, 
that  of  the  cerebellum,  morphologically  formed  from  roof  and 
sides  of  the  metencephalon.     In  some  forms,  as  in  the  adult 
dog-fish,  these  folds,  three  or  four  in  number,  are  seen  with 
the  clearness  of  a  diagram;  in  others,  as  in  adult  birds  and 
mammals,  the  original  creases  become  shallow  by  a  coalescence 
of  the  applied  surfaces  of  adjacent  folds,  but  the  structure  is 
still  marked  by  the  characteristic  dendritic  arrangement  of  the 
white  matter,  which  marks  the  core  of  each  fold.     In  some 
cases  a  secondary  or  even  tertiary  folding  is  thus  marked. 

4.  Flexure,  or  the  bending  upon  itself  of  the  entire  longi- 
tudinal axis  of  the  neural  tube    (Fig.    116).     The   possible 
flexures  are  three  in  number,  apical  flexure,  flexure  of  the  pons, 
and  cervical,  the  two  first  appearing  in  birds  and  quadrupedal 
mammals,  the  last  found  only  in  Man,  caused  directly  by  the 
erect  position  and  the  consequent  bending  of  the  skull  over  an 
angle  of  nearly  90°.     The  reason  for  the  other  flexures  is  un- 
doubtedly the  same  as  in  the  case  of  the  foldings  of  the  surface, 
since  by  folding  the  parts  on  themselves  a  larger  brain  may  be 
accommodated  within  the  length  limits  of  a  given  skull.     The 
gradual  formation  of  these  flexures  may  be  well  seen  during 
the  embryonic  development  of  a  bird  or  mammal,  preferably, 
however,  in  Man,  in  which  alone  the  third  or  cervical  flexure 
is  involved.     They  occur  in  the  order  of  their  position,  begin- 
ning anteriorly,  the  first  being  dorsal,  the  second  ventral,  and 
the  third  dorsal  again,  in  accordance  with  the  natural  law  of 
folded  objects. 

With    the    above   principal    in   mind,    the    further   history 


PLATE  IV.  Longitudinal  median  sections  of  Vertebrate 
brains  corresponding  to  the  first  half  of  the  series  in  Fig.  117  in 
the  text,  [(b)  and  (c)  after  EDINGER.] 

(a).  Selachian;    (b).  Teleost;    (c).  Amphibian. 

Color  Scheme  -.yellow,  telencephalon;  blue,  diencephalon;  red,  mesenceph- 
alon;  green,  metencephalon;  brown,  myelencephalon  and  cord. 


r~i£> 


-\-      "', /^N 


PLATE  V.  Longitudinal  median  sections  of  Vertebrate  brains 
corresponding  to  the  second  half  of  the  series  in  Fig.  117.  [After 
EDINGER.] 

(d).  Reptile;     (e).  Bird;    (f).  Mammal. 

Color  Scheme:  yellow,  telencephalon;  blutt  diencephalon  ;  red,  mesenceph- 
alon;  green,  metencephalon;  brmvn,  myelencephalon  and  cord. 


THE    NERVOUS    SYSTEM 


415 


of  the  development  of  the  five  areas  of  the  brain,  as  shown  in 
the  different  vertebrate  Classes,  may  be  studied  by  the  help  of 
the  accompanying  diagrams  [Plates  IV  and  V],  which  rep- 
resent the  adult  brains  of  a  dog-fish,  a  teleost,  an  amphibian,  a 
reptile,  a  bird  and  a  mammal,  sectioned  sagittally  in  the  median 
plane  and  viewed  from  the  inner  aspect.  The  comprehension 


FIG.  116.    Diagram  of  the  cerebral  flexures. 

Angle  between  axes  ab  and  cd  —  apical  flexure.  Angle  between  axes  cd  and 
ef  •=  flexure  of  the  pons.  Angle  between  ef  and  gh  =  cervical  flexure. 

of  these  will  be  facilitated  by  comparing  them  with  Fig.  117, 
which  shows  the  dorsal  aspect  of  the  same  series. 

The  telencephalon  in  the  dog-fish  is  inconspicuous  in  size  but 
of  a  considerable  thickness,  which  is  approximately  uniform 
save  at  the  posterior  part  of  the  roof,  where  a  considerable 
area  retains  its  membranous  character  and  invaginates  to  form 


416  HISTORY   OF    THE    HUMAN    BODY 

three  chorioid  plexuses,  one  for  each  telencephalic  lobe  and  one 
for  the  diencephalon  (third  ventricle).  The  thickening  in  the 
floor  forms  the  area  to  be  known  later  as  the  corpus  striatum; 
that  of  the  roof  and  sides  is  the  potential  cerebrum.  The  two 
telencephala  thus  represent  the  cerebral  hemispheres  together 
with  the  corpora  striata ;  their  cavities  are  the  lateral  ventricles 
in  which  lie  the  two  plexus  chorioides,  the  entrance  of  which 
into  the  ventricles  is  effected  through  the  interventricular 
foramina.  The  anterior  portion  of  each  telencephalon  forms 
an  extensive  olfactory  lobe  (rhinencephalon),  which  is  here 
voluminous  and  stalked.  This  latter  portion  is  in  reality  noth- 
ing less  than  the  "  olfactory  nerve,"  which,  when  stalked  as 
here,  and  especially  when  prolonged,  as  in  some  lizards  and  in 
birds,  gives  the  appearance  of  a  true  cranial  nerve.  It  is  here 
seen  not  to  be  a  genuine  cranial  nerve,  but  an  element  of  the 
brain. 

The  teleosts  and  ganoids  show  a  unique  development  of  this 
part;  the  entire  roof  and  sides  remain  membranous,  but  the 
corpora  striata  are  enormously  developed.  Since  the  mem- 
branous portion,  which  is  here  called  the  pallium,  or  mantle, 
is  absolutely  transparent  and  extremely  delicate,  it  is  usually 
lost  in  dissection,  or  if  retained,  seems  of  no  importance ;  and 
as  the  corpora  striata  are  very  large  and  convex,  they  seem  to 
the  casual  observer  to  be  the  true  cerebral  hemispheres.  The 
rhinencephala  are  well  developed  and  appear  as  the  direct  con- 
tinuation of  these  latter  parts. 

The  telencephalon  of  amphibians  and  reptiles  is  not  unlike 
that  of  the  selachians  (dog-fish),  of  which  it  seems  a  direct 
descendant.  The  rhinencephalon  is  proportionately  smaller, 
although  in  many  lizards  it  becomes  greatly  extended,  in  adap- 
tation to  the  prolonged  snout. 

In  birds  there  is  again,  as  in  teleosts,  an  enormous  develop- 
ment of  the  corpora  striata,  which  makes  up  the  bulk  of  the 
cerebrum,  although  the  roof  and  sides  have  some  thickness  and 
are  not  reduced  to  the  condition  of  a  pallium.  In  the  mammals 
the  telencephalon  reaches  its  highest  development,  when  it  usu- 
ally greatly  exceeds  in  bulk  the  remainder  of  the  brain.  This 


THE    NERVOUS    SYSTEM  417 

excessive  development  is  mainly  that  of  the  roof  and  outer  side 
of  each  of  the  telencephalic  lobes,  which  form  enormous  hemi- 
spheres that  extend  forward  over  the  rhinencephalon  and 
backward  over  di-  and  mesencephalon,  usually  coming-  almost 
in  contact  with  the  cerebellum  (metencephalon),  from  which 
they  are  separated  merely  by  a  membranous  or  bony  par- 
tition, the  tentorium.  In  addition  to  increase  in  bulk  there 
is  also  an  important  histological  change,  namely,  the  appear- 
ance of  large  masses  of  ganglion  cells  over  the  outer  surface, 
arranged  in  definite  layers  and  constituting  the  most  important 
nervous  element,  the  seat  of  the  highest  faculties.  This 
ganglionic  tissue  forms  a  definite  layer  of  gray  matter  of  con- 
siderable thickness,  the  cortex  cerebri.  In  lower  mammals, 
such  as  the  marsupials  and  rodents,  the  outer  surface  of  the 
hemispheres  remains  smooth,  but  in  the  higher  Orders,  such  as 
the  ungulates,  the  carnivores,  and  especially  the  primates,  it 
becomes  folded  up  into  irregular  rounded  elevations,  the  gyri 
or  convolutions,  separated  from  one  another  by  grooves,  the 
deeper  of  which  are  termed  fissures,  (e.  g.t  lateral  cerebral  fis- 
sure [fissure  of  Sylvius] )  ;  and  the  others,  sulci  (e.  g.,  sulcus 
centralis  [fissure  of  Rolando]). 

This  folding  of  the  surface  has  the  evident  effect  of  still 
further  increasing  the  physiological  efficiency  of  the  cerebral 
cortex  by  extending  its  surface  area  within  the  same  mass 
limits. 

While  the  main  mass  of  the  hemispheres  is  derived  from  the 
roof  and  the  outer  side  of  the  telencephalic  lobes,  the  inner  side, 
remaining  thin  at  first,  makes  a  contribution  in  the  form  of  a 
longitudinal  imagination  which  thickens,  and  forms  a  ridge 
that  encroaches  upon  the  lateral  ventricle.  This  is  the  hippo- 
campus [hippocampus  major  or  Amman's  horn}.  It  attains  a 
considerable  development  in  Man,  where  it  forms  a  conspicu- 
ous elevation  upon  the  inner  side  of  the  floor  of  the  ventricle 
and  becomes  prolonged  posteriorly  into  a  free  rounded  end, 
terminating  in  digitations  [pes  hippocampi].  This  intrudes 
itself  upon  the  thick  outer  portion  and  lies  imbedded  in  it, 
covered  by  the  temporal  lobe. 


4i8  HISTORY    OF    THE    HUMAN    BODY 

Still  further  down,  ventral  to  the  hippocampus,  and  partly 
enclosed  by  the  surrounding  parts,  the  same  inner  walls  of  the 
two  hemispheres  come  in  contact  and  form  a  thin  double  par- 
tition known  as  the  septum  pellucidum.  The  two  walls  en- 
close a  small  space  to  which  the  name  "fifth  ventricle"  was 
formerly  given.  It  is  unnecessary  to  state  that  this  is  not  a 
true  ventricle  and  has  no  connection  with  the  lumen  of  the 
neural  tube. 

The  telencephalon  of  all  higher  mammals  is  further  distin- 
guished by  the  formation  of  an  extensive  bridge  or  commissure 
across  the  middle  line  between  the  two  lobes.  This  lies  dorsal, 
and  is  easily  seen  by  drawing  the  hemispheres  a  little  apart 
and  looking  down  from  above.  It  is  called  the  corpus  callo- 
sum,  and  consists  of  fibers  of  white  matter  that  form  a  me- 
dium of  intercommunication  between  corresponding  parts  of 
the  two  hemispheres  and  insure  harmony  of  action.  Aside 
from  this  extensive  commissural  system,  which  has  evidently 
arisen  in  mammals  in  connection  with  the  added  needs  coming 
from  larger  hemispheres,  there  are  three  smaller  transverse 
bundles,  common  also  to  the  brain  of  lower  forms,  the  ante- 
rior, middle  and  posterior  commissures.  Of. these  the  first 
alone  comes  within  the  province  of  the  telencephalon,  the  others 
are  respectively  di-  and  mesencephalic. 

The  diencephalon,  never  an  extensive  element  in  the  verte- 
brate brain,  becomes  nearly  or  wholly  covered  dorsally  and 
laterally  in  the  higher  forms  by  the  excessive  development  of 
other  parts,  but  though  small  and  of  subordinate  interest  in 
itself,  it  is  especially  characterised  by  the  formation  of  second- 
ary organs,  either  as  in-  or  out-pushings.  Some  of  these  latter 
become  of  fundamental  importance  while  others  appear  to  be 
more  or  less  vestigial,  presumably  inherited  from  preverte- 
brate  ancestors  and  of  problematic  significance. 

Several  of  these  formations  occur  along  the  dorsal  aspect^ 
where  over  a  considerable  area  of  debatable  territory  between 
tel-  and  di-encephala  the  roof  remains  thin.     The  most  an- 
terior consists   of  an   extensive  invagination   into   the  third 
ventricle,  which  lies  just  beneath  this  region.     This  invagina- 


THE     NERVOUS     SYSTEM 


419 


tion  is  accompanied  by  blood-vessels,  and  by  division  forms 
three  chorioid  plexuses,  a  median  one  for  the  third  ventricle 
(the  tela  chorioidea  of  human  anatomy)  and  the  two  lateral 


d  e  f 

FIG.  117.  Dorsal  views  of  vertebrate  brains,  corresponding  to  the  lon- 
gitudinal sections  given  in  plates  IV  and  V. 

(a)  Selachian  (dog-fish),  (b)  Teleost  (sculpin).  (c)  Amphibian  (frog.)  (d)  Rep- 
tile (turtle),  (e)  Bird  (sparrow),  (f)  Mammal  (cat). 

I,  telencephalon;  II,  diencephalon;  III,  mesencephalon ;  IV,  metencephalon; 
V,  myelencephalon.  In  (f)  cerebrum  and  cerebellum  have  been  drawn  apart  to 
expose  the  mid-brain. 


420  HISTORY   OF   THE    HUMAN    BODY 

ones  already  mentioned  (tcenice  chorioides),  which  pass 
through  the  interventricular  foramina  and  supply  the  two 
lateral  ventricles  of  the  telencephalon. 

Behind  the  plexuses  there  appear  in  the  mid-dorsal  line  typi- 
cally two  median  diverticula,  which,  owing  to  the  many  grades 
of  development  under  which  they  appear,  as  well  as  to  the  fact 
that  they  have  long  been  treated  as  identical,  have  received  a 
large  number  of  distinct  designations.  The  more  anterior  of 
these  is  best  known  as  the  paraphysis,  the  posterior  one  the 
epiphysis,  but  the  former  is  also  correctly  known  as  the  parietal 
organ,  the  latter  as  the  pineal  organ.  Both  show  a  tendency 
to  pass  through  the  skull  and  attain  a  position  directly  beneath 
the  skin  in  the  middle  line,  developing  there  a  rudimentary 
sense  organ  of  uncertain  nature,  but  probably  an  eye  in  each 
case. 

In  the  cyclostome  Petromyzon,  both  structures  attain  consid- 
erable development,  and  the  optical  structure  of  the  epiphysial 
organ  is  evident  through  the  occurrence  of  pigment  in  what 
may  be  well  a  vestigial  retina.  The  paraphysial  organ  is 
smaller,  but  of  similar  structure.  In  no  other  form  are  both 
of  these  structures  so  well  developed,  but  in  several  cases  one 
may  attain  an  even  higher  development  while  the  other  is  rudi- 
mentary. In  some  instances  the  highest  point  in  development 
is  reached  during  embryonic  life,  while  in  others  it  is  exhibited 
by  the  adult.  Thus  in  the  selachians,  the  epiphysis  passes 
through  a  minute  foramen  in  the  skull  and  reaches  the  surface ; 
its  terminal  organ  is  visible  externally,  but  the  paraphysis  is 
not  developed  at  all.  In  frogs  and  toads  the  paraphysis  attains 
a  development  similar  to  that  of  the  epiphysis  in  the  former 
case,  while  this  latter  part  has  not  been  found.  The  para- 
physial organ,  here  known  as  the  "  frontal  organ,"  is  plainly 
visible  externally,  but  in  the  adult  is  entirely  separated  from 
the  brain  by  the  retrogression  of  its  stalk.  The  highest  de- 
velopment of  either  organ  is  reached  among  certain  lizards, 
where  it  is  the  epiphysis  that  is  thus  favored.  The  terminal 
organ  here  lies  in  a  socket  (parietal  foramen)  formed  in  the 
interparietal  suture  and  represents  a  fairly  good  eye,  with  pig- 


THE   NERVOUS    SYSTEM 


421 


mented  retina,  a  more  or  less  makeshift  lens,  and  a  well-devel- 
oped nerve  connecting  the  terminal  organ  with  the  brain. 
Above  this,  on  the  surface,  is  situated  a  transparent  scale, 
surrounded  by  a  ring  of  smaller  opaque  ones,  making  a  con- 


FIG.   118.     Lateral  views  of  the  developing  human  brain;  drawn  from 
wax  models  by  F.  ZIEGLER  after  WM.  His. 

The    cranial    nerves    are    indicated    by    roman    numerals;    exponent    letters    m    and 
s    denotes    respectively    motor    and    sensory    branches;    e,    the    otic    vesicle 

spicuous  object  on  the  heads  of  these  forms.  The  paraphysis 
appears  to  be  associated  with  this  epiphysial  structure.  In 
birds  and  mammals  there  seems  to  be  no  trace  of  a  paraphysis, 
while  the  epiphysis  is  reduced  to  the  form  of  the  so-called 
"pineal  gland,"  pushed  backwards  from  its  original  position 


422  HISTORY   OF    THE    HUMAN    BODY 

by  the  growth  of  other  parts.  In  man  it  lies  so  hidden  that  the 
early  anatomists,  finding-  it  as  it  were  in  the  innermost  pene- 
tralia of  the  organ  of  life  and  individuality,  deemed  it  the  seat 
of  the  soul,  a  view  from  which  the  morphologists  of  the  pres- 
ent day  have  escaped  only  by  substituting  one  mystery  for 
another. 

The  sporadic  occurrence  of  these  vestigial  sense  organs, 
paraphysis  and  epiphysis,  which,  save  perhaps  in  the  case  of 
the  parietal  (epiphysial)  eye  of  the  lizard,  cannot  be  of  the 
slightest  use,  points  definitely  to  the  presence  of  similar  organs 
in  a  functional  condition  in  some  remote  ancestor.  That  these 
parts  were  organs  of  vision  there  can  be  but  little  doubt,  and 
there  are  certain  indications  which  lead  us  to  think  that  they 
were  once  paired,  although  always  close  together.  Beyond 
this,  investigation  has  as  yet  shown  nothing,  and  the  whole 
subject  remains  at  present  one  of  those  half-completed  histo- 
ries, of  which  the  record  consists  of  a  few  poorly  preserved 
fragments. 

Far  more  satisfactory  is  the  history  of  the  diverticula  which 
develop  laterally  from  the  sides  of  the  part  under  consideration, 
for,  although  we  do  not  have  adult  animals  which  show  the 
steps  in  the  development,  they  are  yet  traced  in  perfect  agree- 
ment during  the  embryological  history  of  every  vertebrate,  a 
procedure  familiar  to  all  students  of  embryology.  These  appear 
at  an  extremely  early  age,  often  beginning  before  the  com- 
pletion of  the  telencephalic  lobes,  and  soon  assume  the  form  of 
spherical  vesicles,  connected  with  the  brain  by  narrow  stalks, 
and  almost  in  contact  at  their  outer  surface  with  the  external 
germ-layer,  the  surface  ectoderm.  By  an  invagination  of  this 
outer  surface  the  vesicle  is  transformed  into  a  double-layered 
cup,  and  in  this  one  may  recognize  the  fundamental  elements  of 
the  eye.  The  primary  vesicle  is  hence  called  the  optic  vesicle, 
the  transformed  cup-like  figure,  the  optic  cup.  [See  Fig.  136.] 

Of  this  the  invaginated  layer,  now  lining  the  cup,  becomes 
the  retina,  certain  cells  of  which  give  rise  to  the  rods  and  cones, 
the  essential  nervous  elements  of  the  organ ;  the  other  layer, 
now  forming  the  covering  of  the  cup,  develops  pigment  and 


THE   NERVOUS    SYSTEM  423 

becomes  the  tapetiun  nigrum,  a  layer  which,  together  with  the 
blood  capillaries  later  to  be  associated  with  it,  will  become  the 
chorioid  coat.  The  stalk,  although  not  directly  transformed 
into  the  optic  nerve,  forms  the  path  along  which  it  develops 
and  thus  marks  its  final  position.  (For  the  details  of  this 
cf.  the  last  part  of  Chapter  XL) 

During  the  time  at  which  the  optic  cup  has  been  forming  by 
a  turning  in  of  the  outer  part  of  the  vesicle,  an  associated 
process  takes  place  in  the  ectoderm  directly  opposite  the  cup, 
This  process  consists  of  an  inpushing  from  without  on  the  part 
of  this  ectoderm,  the  inpushing  going  rapidly  through  the 
stages  of  a  simple  depression,  a  depression  with  a  narrowed 
neck,  and  finally  that  of  a  spherical  vesicle  entirely  cut  off  from 
its  layer  of  origin.  That  this  may  once  have  been  the  essential 
sense  organ  to  supply  the  needs  of  which  the  diverticulum  from 
the  brain  may  have  originated,  seems  likely  from  the  similarity 
of  its  early  development  to  that  of  certain  actual  sense  organs, 
especially  the  otic  capsule,  which  develops  into  the  inner  ear. 
This  latter,  as  will  be  shown  later,  appears  to  have  been  at 
first  merely  a  single  unit  of  the  system  known  as  the  "  lateral 
line  organs,"  and  the  lens,  although  no  longer  sensory  in  func- 
tion, may  with  some  probability  be  referred  to  the  same  source. 
In  all  present-day  vertebrates,  however,  it  is  no  longer  sensory, 
but  develops  into  an  auxiliary  though  essential  organ  of  the 
eye,  the  crystalline  lens.  This  is  accomplished  by  an  enormous 
thickening  of  the  inner  wrall  of  the  vesicle,  which  finally  fills  up 
the  entire  lumen,  leaving  the  outer  wall  to  fit  over  it  in  the 
form  of  a  protecting  epithelium.  During  later  development 
the  eye  receives  its  vitreous  humor,  its  blood-vessels,  its  sclero- 
tic coat  and  other  essential  parts  from  the  surrounding  tissue, 
mainly  the  mesenchyme,  and  develops  into  the  adult  form. 

But  one  other  diverticulum  arises  from  the  diencephalon, 
and  that  one  is  directed  downwards  from  the  middle  of  its 
floor.  Like  the  lateral  eyes,  it  does  not  form  a  complete  organ 
in  itself,  but  unites  with  a  similar  diverticulum  which  develops 
upward  from  the  roof  of  the  mouth,  and  together  they  form  an 
organ  of  slight  functional  importance,  in  respect  to  which  the 


424  HISTORY   OF   THE    HUMAN    BODY 

elaborate  method  of  development,  involving  as  it  does  two  dis- 
tinct elements,  is  disproportional.  It  is  thus  generally  sup- 
posed that  we  have  a  vestigial  organ  like  those  developing  dor- 
sally  and  laterally  from  the  same  region,  and  that  it,  like  them, 
represents  the  remnant  of  an  organ  of  considerable  importance 
in  some  unknown  ancestral  group.  This  organ  is  a  noticeable 
feature  of  the  ventral  aspect  of  all  vertebrate  brains,  and  bears 
the  noncommittal  name  of  hypophysis,  literally  that  which 
grows  beneath,  in  allusion  to  its  position.  In  most  skulls,  es- 
pecially in  the  more  completely  ossified  one  of  the  amniotes, 
there  is  a  distinct  depression  for  its  lodgment  (the  sella  turcica 
of  human  anatomy),  and,  as  the  hypophysis  is  often  connected 
with  the  brain  by  a  narrow  stalk  around  which  the  bone  may 
fit  quite  tightly,  it  is  seldom  removed  in  its  entirety  with  the 
brain,  and  hence  its  true  relations  are  apt  not  to  be  wholly 
understood. 

The  portion  contributed  by  the  diencephalon  is  in  the  form 
of  a  hollow  cone  or  funnel,  the  infundibulum.  About  this  the 
invagination  from  the  mouth  cavity,  which  is  glandular  in  its 
nature,  and  termed  pituitary  body,  becomes  developed,  and  by 
the  secondary  loss  of  the  original  connection  between  this  latter 
and  the  roof  of  the  mouth,  through  the  development  of  the 
palate,  the  hypophysis  is  made  to  appear  like  a  simple  organ, 
attached  to  the  brain. 

Although  there  is  no  feeling  of  certainty  among  morpholo- 
gists  concerning  the  original  form  of  this  organ,  the  opening 
of  the  pituitary  portion  into,  or  rather  from,  the  exterior 
in  the  more  primitive  forms,  suggests  that  this  part  may  repre- 
sent the  rudiment  of  an  earlier  mouth,  the  palceostoma,  with 
which,  as  shown  by  other  data,  the  prevertebrate  ancestors 
seem  to  have  been  equipped  prior  to  the  development  of  the 
definite  vertebrate  mouth,  the  neostoma* 

Aside  from  these  diverticula  and  the  organs  found  in  asso- 

*The  pituitary  diverticulum  arises  in  gnathostomes  from  the  ectoderm 
of  the  stomatodaeal  invagination,  but  in  cyclostomes  is  beyond  the  limits 
of  the  mouth  and  pushes  in  from  the  external  surface  of  the  head  in  close 
association  with  the  medial  nasal  invagination.  For  farther  details  and 
theories  concerning  this  part  see  Chapters  VI,  XI  and  XII;  also  Fig.  129. 


THE   NERVOUS    SYSTEM  425 

ciation  with  them  the  diencephalon  develops  in  mammals  a 
pair  of  lateral  ganglionic  masses,  the  thalami  optici,  which 
arise  as  thickenings  of  the  sides  of  the  vesicle  beneath  the  optic 
stalks.  These  are  to  be  sharply  distinguished  from  the  lobi 
optici  (optic  lobes),  under  which  name  the  lateral  halves  of 
the  mesencephalon  are  usually  described. 

The  mesencephalon  is  the  most  conservative  of  the  elements 
of  the  brain :  it  develops  very  little  that  is  new  throughout  its 
entire  history,  and  in  Man  and  the  other  mammals,  although 
suffering  little  or  no  actual  diminution  in  size,  it  becomes  re- 
duced proportionately  to  a  very  small  portion  of  the  brain 
through  the  excessive  growth  of  the  surrounding  parts.  This 
is  made  clear  by  the  diagrams,  in  which  the  mesencephalon  may 
be  followed  through  fishes,  amphibians  and  reptiles  with  but 
little  change.  Its  roof  and  outer  sides  are  moderately  thick- 
ened and  usually  divided  along  the  mid-dorsal  line  by  a  longi- 
tudinal groove,  thus  forming  a  bilobate  organ,  the  corpora 
bigemina  or  lobi  optici.  In  many  fishes  they  form  a  conspicu- 
ous part  of  the  brain  which,  so  long  as  the  cerebral  lobes  remain 
but  slightly  developed,  must  be  of  great  functional  importance. 
In  some  teleosts,  for  example,  in  which  the  cerebral  hemi- 
spheres are  represented  merely  by  a  non-nervous  membrane, 
they  furnish  at  least  two-thirds  of  the  dorsal  surface,  and  thus 
perhaps  functionally  replace  the  former.  In  amphibians  and 
reptiles  the  gradual  development  of  the  cerebral  hemispheres 
reduces  the  importance  of  the  optic  lobes,  although  in  birds, 
forms  not  in  the  direct  line  of  human  ancestry,  they  again  reach 
a  certain  prominence;  thus  when  the  enormously  developed 
corpora  striata  and  the  small  and  thin  walls  of  the  hemispheres 
are  taken  into  consideration,  birds  are  seen  to  be  as  unique  in 
their  brain  development  as  they  are  in  their  skeleton  and  their 
general  form. 

In  mammals  the  mesencephalon  is  to  be  looked  for  between 
the  two  greatly  hypertrophied  elements,  telencephalon  and 
metencephalon  (cerebrum  and  cerebellum),  and  here  the 
bilobed  organ  has  become  transformed  into  one  with  four 
lobes,  the  corpora  quadrigemina.  The  beginning  of  this 


426  HISTORY    OF    THE    HUMAN    BODY 

change  may  be  found  in  some  reptiles,  where  the  develop- 
ment of  a  pair  of  small  subordinate  lobes  posterior  to 
the  main  ones  makes  it  clear  that  the  four-lobed  form  in 
mammals  is  due  to  the  development  of  a  new  pair  of  lobes 
posterior  to  the  others,  and  not  merely  to  the  formation 
of  a  cross-furrow.  Subordinate  lobes  like  those  of  reptiles  are 
found  also  in  birds. 

The  floor  of  the  mesencephalon  is  thickened  in  all  cases  and 
is  of  considerable  functional  importance.  Through  this  region 
pass  the  fibers  of  connection  between  the  cerebral  lobes  and  the 
medulla,  and  as  the  hemispheres  increase  in  size,  these  bundles 
become  greater  and  form  the  pedunculi  cerebri  [crura 
cerebri},  especially  conspicuous  in  mammals,  as  would  be 
expected. 

During  the  process  of  phylogenetic  development  the  roof 
and  sides  of  the  metencephalon  become  selected  as  a  region 
where  a  large  part  of  the  work  of  the  central  nervous  system 
is  accomplished.  This  part,  the  cerebellum,  is  thus  almost  al- 
ways large  and  voluminous,  and  often,  even  in  fishes,  becomes 
folded  up  into  several  plicae,  thus  emphasizing  its  great  func- 
tional importance. 

It  has  already  been  shown  how  both  the  corpora  striata  and 
the  lobi  optici,  although  of  supreme  importance  in  some  fishes, 
eventually  become,  except  perhaps  in  birds,  entirely  subordi- 
nated to  the  cerebral  hemispheres ;  the  cerebellum,  on  the  other 
hand,  has  retained  from  the  first  an  office  of  great  importance, 
and  in  mammals  becomes  subordinated  to  the  hemispheres 
alone.  There  are  occasional  exceptions  to  the  general  impor- 
tance of  this  part,  as  in  the  case  of  the  singularly  small  cere- 
bellum of  the  frog,  but  such  cases  are  very  few.  The  floor  of 
the  metacephalon  is  utilized  in  part  for  the  location  of  com- 
missural  fibers  between  the  two  lateral  halves  of  the  cerebellum, 
and  in  mammals,  corresponding  to  the  increase  in  size  of  this 
organ,  this  commissural  bundle  becomes  large  and  conspicu- 
ous, forming  a  broad  loop  around  the  base  of  the  medulla,  the 
pons  [ Varoli\\ . 

Although  this  region  of  the  myelencephalon  is  perhaps  the 
most  complex  of  any  part  of  the  brain,  this  complexity  lies  in 


THE    NERVOUS    SYSTEM  427 

the  minute  structure  rather  than  in  the  external  form,  in  which 
latter  respect  it  is  singularly  simple  and  uniform  throughout  all 
vertebrates.  Its  sides  and  floor,  which  alone  come  into  consid- 
eration as  a  nervous  organ,  together  with  the  crura  cerebri  and 
the  pons,  form  the  central  system  of  commissures  for  the  entire 
nervous  system,  receiving  the  fibers  from  all  other  parts  and 
forming  the  necessary  connections  of  these  with  one  another 
and  with  the  spinal  cord.  That  these  connections  become 
vastly  more  complex  in  the  higher  than  in  the  lower  vertebrates 
is  evidenced  both  by  the  gradual  growth  of  the  various  parts 
of  the  brain  in  size  and  complexity,  and  by  the  results  as  seen 
in  the  behavior  of  living  animals.  A  rhomboidal  area  which 
includes  the  greater  part  of  the  roof  of  this  part  remains  mem- 
branous and  forms  an  important  chorioid  plexus,  that  of  the 
fourth  ventricle  (tcenia  ventriculi  quarti).  As  this  thin  place 
and  its  subjacent  rhomboid  cavity  (fossa  rhomb oidalis)  are 
extremely  conspicuous  objects  in  all  embryos,  this  portion 
of  the  brain  is  often  conveniently  termed  the  rhomben- 
cephalon. 

Morphologically  the  medulla  is  the  anterior  continuation  of 
the  spinal  cord,  and  the  nerves  that  proceed  from  it  resemble 
the  spinal  nerves  more  than  do  those  which  arise  farther  for- 
ward. In  fact  the  line  of  division  between  medulla  and  cord 
is  an  artificial  one,  the  first  being  considered  as  coterminous 
with  the  skull  in  all  cases.  Similarly  those  nerves  in  that 
region  which  obtain  their  exit  through  a  foramen  in  the  skull 
are  termed  cranial  and  are  accorded  to  the  medulla.  The  ar- 
tificial character  of  this  distinction  involves  confusion  at  one 
point  at  least,  namely,  the  varying  limits  of  the  skull  between 
amphibians  and  reptiles  due  to  the  absorption  of  a  vertebra. 
(See  Chap.  V.)  In  this  way  the  hypoglossal  nerve  (Xllth), 
a  spinal  nerve  in  amphibians,  becomes  added  to  the  list  of 
cranial  nerves  in  the  Amniota,  although  this  case  involves 
rather  more  than  the  simple  addition  of  a  single  pair  of  spinal 
nerves,  and  is  still  a  somewhat  obscure  point. 

Beyond  the  medulla  the  neural  tube  becomes  the  spinal  cordf 
which,  although  it  often  shows  some  little  regional  differentia- 
tion, is  far  more  conservative  than  the  anterior  portion  and 


428  HISTORY   OF   THE    HUMAN    BODY 

consists  essentially  in  all  cases  of  a  tube  with  a  minute  lumen 
(canalis  centralis)  and  extremely  thick  walls.  It  consists  of 
both  ganglion  cells  (gray  matter)  and  connecting  fibers  (white 
matter)  and,  as  the  latter  usually  form  the  greater  part  of  its 
bulk,  it  is  to  be  considered  in  the  main  a  great  central  nerve 
bundle  proceeding  from  the  brain  and  distributing  its  fibers  to 
all  parts.  This  distribution  takes  place  through  the  formation 
of  pairs  of  spinal  nerves,  which  are  arranged  metamerically,  a 
pair  for  each  body  somite. 

The  proportion  of  the  spinal  cord  in  weight  as  compared 
with  that  of  the  brain  may  be  said  in  a  general  way  to  decrease 
as  we  ascend  the  scale  of  vertebrates,  but  this  is  due  rather  to 
the  increase  in  the  size  of  the  brain  than  to  a  decrease  in  that 
of  the  cord.  There  is,  however,  another  principle,  that  of  pro- 
gressive cephalization,  which  tends  to  shorten  the  cord  and  con- 
centrate the  nervous  system  at  the  anterior  end,  and  it  is 
through  this  that  the  changes  may  be  best  explained.  This 
principle  appears  equally  well  among  many  groups  of  inverte- 
brates and  is  shown  in  ( i )  )  a  tendency  to  shorten  the  body 
axis,  and  (2)  to  concentrate  and  hence  shorten  the  longitudinal 
nerve  axis.  The  results  of  this  process  may  be  especially  well 
followed  among  such  a  group  of  animals  as  that  of  insects,  in 
which  the  central  nervous  system  originally  consists  of  a  pair 
of  small  ganglia  for  each  somite,  this  condition  running 
through  the  entire  body. 

Thus  in  the  myriapod  (Fig.  119,  A),  an  ancestral  form,  the 
primitive  condition  is  still  realized ;  in  such  a  low  form  of  insect 
as  the  dragon  fly  or  grasshopper  the  concentration  of  ganglia 
has  commenced,  and  in  the  fly  the  highest  cephalization  is 
reached.  That  these  stages  are  passed  through  during  the  de- 
velopment is  shown  by  a  comparison  of  the  nervous  system  of 
the  fly  in  its  various  stages,  that  of  the  larva  still  showing 
a  quite  primitive  condition  (Fig.  119,  cf.  B  and  C). 

This  principle  is  shown  in  vertebrates  by  the  progres- 
sive shortening  of  the  spinal  cord  in  a  series  of  gradually  ceph- 
alizing  forms,  but  can  be  used  as  a  criterion  of  development 
only  within  the  limits  of  a  single  group.  Thus  the  frog,  with 


THE   NERVOUS    SYSTEM 


429 


its  extreme  shortening-  of  the  cord,  exhibiting  but  ten  pairs  of 
spinal  nerves,  is  not  to  be  compared  with  Man,  in  which  there 
are  more  than  thirty,  but  with  the  long-bodied  salamanders, 


B 


FIG.  119.  Nervous  systems  of  invertebrates,  showing  the  principle  of 
concentration.  [A,  from  LANG,  after  OUDEMANS;  B  and  C  after  LOWNE.] 

(A)  Nervous  system  of  the  myriapod  Lithobius,  showing  a  connected  chain  of 
approximately  equal  ganglia.  (B)  Nervous  system  of  the  larval  Chironomus,  the 
"  harlequin  fly,"  showing  a  long  chain  of  ganglia  as  in  A.  (C)  Nervous  system 
of  the  adult  Chironomus,  with  all  the  ganglia  concentrated  into  two,  cephalic  and 
mid-thoracic. 


430 


HISTORY    OF    THE    HUMAN    BODY 


animals  in  its  own  class,  in  which  the  cord  is  nearly  coterminous 
with  the  tail  (Fig.  120).  Indeed,  an  almost  absurd  result  of 
this  is  shown  in  a  certain  teleost,  Orthagoriscus  (Fig.  121,  b), 


a 


Brach 


H 


ll/hy 


Cm. 

Sciat 


FIG.  120.     Spinal  nerves  of  two  amphibians,  showing  differences  in  the 
degree  of  concentration. 

(a)    Frog.     [After  GAUPP.]     (b)    Necturus.     [Combined  from  drawings  by  WAITE.] 
Hpgl,   hypoglossal   nerve;   Brach.   brachial;    Thor.   abd,   thoraco-abdominal ;   Sp.   cor, 

supracoracoid ;    //.    hy,    ilio-hypogastric ;     Cru.    crural;    Sciat,    sciatic.      The    vertebrae 

are  numbered  by  arabic  numerals,  the  spinal  nerves  by  roman. 


THE    NERVOUS    SYSTEM 


in  which  the  entire  cord  is  perhaps  a  little  shorter  than  the 
brain.  In  all  cases  in  which  a  shortening  has  occurred  two  con- 
nected   phenomena    may    be 
observed  at  the  posterior  end 
of  the  cord:  first,  the  thick, 
functional  portion  terminates 
more  or  less  abruptly  and  the 
cord  is  continued  as  a  taper- 
ing   thread    known    as    the 
filum      terminate,      without 
function  as  a  nervous  organ, 
and  secondly,  the  shortening 
is  usually  so  great  that  the 
posterior  portion  of  the  cord 
is    drawn    up    considerably 
ahead  of  the  parts  which  it 
supplies,       compelling       the 
nerves     involved     to     turn 
around    at    a    progressively 
sharper  angle  until  the  most 
posterior    ones     run     in    a 
longitudinal  direction  paral- 
lel  to   the   filum   terminale. 
This  bundle  of  approximate- 
ly longitudinal  nerves  which 
appears    thus    to    terminate 
the  cord,  is  known  collective- 
ly as  the  cauda  equina,  and  is 
often  a  noticeable  object,  as 
in    the    frog    and    in    Man 
(Fig.  121,  c).     In  the  higher 
vertebrates,      especially      in 
mammals,    the    relation    of 
spinal   cord  to  tail  becomes 
quite  different  from  that  of 
the   lower  forms,   a  change 
that  is  correlated  in  an  inter- 


FIG.  121.  Spinal  cords,  show- 
ing the  intumescentia,  also  a 
marked  length  variation. 

(a)  Turtle.  [After  BOJANUS.]  (b) 
Orthagoriscus  (telecost).  [From  GEGEN- 
BAUR  after  B.  HALLER.]  (c)  Human 
spinal  cord  without  the  brain.  [After 

WlEDERSHEIM.] 


432  HISTORY   OF    THE    HUMAN    BODY 

esting  way  with  changes  in  the  musculature  of  that  part.  In 
such  forms  as  fishes  and  salamanders,  the  metameric  mus- 
culature is  not  discontinued  at  the  cloaca,  which  marks  the 
posterior  limit  of  the  body  cavity,  but  is  continued  in  a  grad- 
ually reducing  series  to  the  extreme  tip  of  the  tail.  Each  of 
these  caudal  metameres  is  supplied  with  a  pair  of  spinal  nerves, 
to  furnish  which  the  cord  must  of  necessity  be  continued  quite 
or  nearly  to  the  end.  In  mammals,  although  in  some  cases  the 
tail  is  long  and  extensive,  its  metameric  muscles  have  been 
given  up  except  those  of  its  most  anterior  somites,  and  the  tail 
is  moved  by  a  complex  system  of  tendons  proceeding  from 
these  latter.  The  only  nerves  necessary  for  the  tail,  then,  are 
those  of  its  anterior  metameres,  which  are  easily  supplied 
from  the  cauda  equina,  thus  obviating  all  necessity  on  the  part 
of  the  cord  for  extending  very  far  posteriorly.  Indeed,  with 
the  exception  of  the  primitive  Ornithorhynchus  and  a  few 
rodents,  the  spinal  cord  of  mammals  fails  to  reach  even  the 
sacrum. 

In  much  the  same  way  as  the  development  of  the  caudal 
muscles  conditions  the  point  and  manner  of  termination  of  the 
cord,  so  is  its  caliber  modified  in  other  places  through  the  rela- 
tive amount  of  muscular  development  in  the  various  body 
regions,  especially  in  the  case  of  the  limbs.  In  such  a  form  as 
Amphioxus,  where  the  successive  metameres  are  practically 
alike,  the  spinal  nerves,  and  consequently  the  cord,  are  of  about 
an  equal  caliber  throughout  the  body,  gradually  tapering  to  the 
end  of  the  tail,  and  for  the  same  reason  in  forms  like  eels  and 
snakes,  which  have  secondarily  lost  their  metameric  differenti- 
ation, the  cord  is  correspondingly  simple;  where,  however,  a 
certain  metamere,  or  a  series  of  successive  ones,  becomes 
greatly  developed,  the  nerves  which  supply  this  part  are  neces- 
sarily increased  in  caliber,  and  this  causes  a  corresponding  in- 
crease in  the  cord  at  or  near  the  point  from  which  they 
originate. 

The  most  conspicuous  example  of  this  principle  is,  of  course, 
that  of  the  limbs,  which  are  often  excessively  developed  in 
the  higher  forms  and  cause  a  corresponding  increase  of  size  in 


THE    NERVOUS    SYSTEM  433 

the  metameric  nerves  from  which  they  receive  their  supply,  as 
well  as  in  the  cord  of  the  region  from  which  these  nerves  pro- 
ceed. It  will  be  remembered  that  each  vertebrate  limb  above 
those  of  the  fishes  is  a  development  of  a  few  (not  more  than 
5-6)  metameres,  and  thus  involves  primarily  a  corresponding 
number  of  nerves.  There  are  thus  in  the  spinal  cord  two 
swellings  (inhtmescentitp) ,  cervical  and  lumbar,  corresponding 
respectively  to  the  anterior  and  posterior  limbs.  These  swel- 
lings are  directly  proportionate  to  the  amount  of  development 
in  each  pair  and  are  markedly  unequal  in  such  forms  as  bats, 
with  their  exaggerated  fore  limbs  and  reduced  hinder  pair, 
and  in  the  ostrich,  in  which  the  development  shows  the  re- 
verse tendency. 

In  snakes,  in  which  the  limbs  have  been  lost,  the  intu- 
mescentise  are  also  absent ;  on  the  other  hand,  in  turtles,  mem- 
bers of  the  same  class,  the  disappearance  of  the  most  of  the 
trunk  muscles  has  caused  a  considerable  reduction  in  the  size 
of  the  cord  between  the  intumescentiae,  making  the  latter, 
which  are  well  developed,  seem  still  greater  by  contrast  (Fig. 
121,  a). 

The  most  exaggerated  development  of  these  spinal  intu- 
mescentise  seems  to  have  been  among  the  extinct  dinosaurs,  in 
which  the  excessive  development  of  the  hind  limbs,  on  which 
they  supported  their  enormous  weight,  caused  a  proportionate 
exaggeration  of  the  corresponding  swelling,  the  intumescentia 
lumbalis.  As  shown  by  the  cavities  in  the  vertebrae  (neural 
canal),  this  intumescence  was  often  considerably  larger  than 
the  entire  brain,  exceeding  that  organ  some  twelve  times  in 
Stegosavrus. 

In  shape,  as  seen  best  by  cross-sections,  the  spinal  cord  varies 
somewhat  in  the  different  regions  of  the  body,  and  consider- 
ably more  in  the  various  vertebrates,  especially  the  lower  as 
compared  with  the  higher.  In  the  cyclostomes  it  is  strongly 
flattened,  convex  dorsally  and  concave  ventrally.  In  am- 
phibians it  is  elliptical,  flattened  from  above  downwards,  and 
with  a  noticeable,  though  not  very  deep,  ventral  furrow.  In 
mammals,  by  the  addition  of  a  dorsal  and  two  lateral  furrows, 


434  HISTORY    OF    THE    HUMAN    BODY 

all  longitudinal  and  parallel,  the  well-known  form  of  a  fluted 
column  is  produced,  with  a  dorsal,  lateral  and  ventral  column 
upon  each  side  [posterior,  lateral,  anterior,  BNA],  The 
shape  of  the  mass  of  gray  matter  as  seen  in  section  varies  from 
that  of  a  symmetrical  triangle  in  lower  forms  to  that  of  a  figure 
like  a  double  crescent  in  the  higher;  in  all  cases  it  retains 
the  primitive  position,  bordering  the  lumen  of  the  tube,  the 
original  external  surface. 

All  parts  of  the  body  are  in  constant  communication  with  the 
central  nervous  system  through  the  medium  of  the  peripheral 
nerves,  which  are  in  structure  essentially  the  same  as  the  white 
matter  of  the  brain  and  cord,  as  seen  in  the  various  commis- 
sures of  the  former  and  in  the  columns  of  the  latter,  save  that 
here  there  is  added  a  connective  tissue  element,  which  not  only 
forms  an  external  sheath  for  each  entire  nerve  (perineurium), 
but  also  a  delicate  wrapping  about  each  nerve  fiber  (neuri- 
lemma).  These  nerves  issue  in  pairs  from  both  brain  and 
cord,  and,  although  in  form  and  character  the  transition  from 
one  group  of  nerves  to  the  other  is  a  gradual  one,  the  two 
groups  are  distinguished  for  convenience  as  cranial  and  spinal. 

The  latter,  which  are  the  less  modified,  issue  from  the  cord 
at  approximately  equal  intervals  and  are  metameric  in  arrange- 
ment, a  pair  corresponding  to  each  metamere  or  body  somite, 
as  expressed  in  the  muscles  or  the  skeletal  parts.  This  meta- 
meric arrangement,  which  is  often  expressed  with  great  clear- 
ness in  the  trunk,  is  not  distinct  in  the  head,  and  the  cranial 
nerves,  although  showing  indications  of  a  former  metameric 
order,  cannot  be  satisfactorily  resolved  into  their  separate  ele- 
ments. 

According  to  their  use  nerves  are  sharply  divided  into  two 
groups,  sensory  and  motor.  The  first  are  distributed  chiefly 
to  the  external  surface  and  are  the  media  by  which  the  central 
organ  receives  intelligence  concerning  external  stimuli.  These 
terminate  in  many  cases  in  special  sense  organs  arranged  to 
receive  certain  definite  stimuli,  but  are  distributed  also  over 
the  general  surface,  where  they  respond  to  simple  contact. 
Other  sensory  nerves  supply  certain  internal  parts,  as  the 


THE    NERVOUS    SYSTEM  435 

muscles,  giving  these  parts  some  degree  of  sensation.  In  all 
sensory  nerves  the  impulse  necessarily  travels  from  the  termi- 
nus to  the  central  organ,  and  these  nerves  are  consequently 
designated  as  centripetal  or  afferent.  The  other  type  of  nerve, 
the  motor,  supplies  the  muscles  and  furnishes  them  with  the 
impulse  to  contract.  In  these  nerves  the  current  runs  from 
the  center  to  the  terminus,  and  they  are  thus  centrifugal,  or 
efferent.  The  nerves  that  regulate  the  action  of  other  organs, 


Dorsal 


VENTRAL 


Ventral 

FIG.  122.    Diagram  of  a  typical  spinal  nerve. 

such  as  the  secretion  of  glands,  are  a  subdivision  of  the  motor 
class. 

Nerves  issue  from  the  central  organ,  whether  brain  or  cord, 
in  bundles  called  roots,  each  of  which  contains  mainly  one 
type  of  nerve  fiber.  The  roots  are  hence  called  either  motor 
or  sensory,  but  since  a  given  part  must  usually  be  supplied  with 
both  motion  and  sensation,  two  roots,  one  of  each  sort,  become 
associated  together,  and  blend  their  fibers  within  a  single  ex- 
ternal sheath,  thus  forming  a  mixed  nerve.  This  is  true  of  all 
of  the  spinal  nerves  and  of  some  of  the  cranial  pairs,  but 
others  of  this  latter  class  arise  from  single  roots  and  retain 
their  simple  character.  Each  metamere  possesses  typically  a 
pair  of  sensory  and  a  pair  of  motor  roots,  the  sensory  situated 
dorsally,  the  motor  ventrally,  thus  forming  two  longitudinal 


436  HISTORY   OF   THE    HUMAN    BODY 

rows  on  each  side.*  The  two  roots  of  the  same  side  unite 
soon  after  their  exit  from  the  cord  into  a  single  metameric 
nerve,  containing  both  sorts  of  fibers,  but  then  divides  again 
almost  immediately  into  dorsal  and  ventral  branches,  each  con- 
taining both  sorts  of  fibers.  These,  like  all  subsequent  di- 
visions, are  merely  topographical,  and  not  physiological,  as  in 
the  case  of  the  roots.  A  spinal  ganglion  appears  in  association 
with  each  pair  of  roots,  usually  associated  with  the  sensory 
root,  but  in  lower  forms  often  connected  with  both  and  situ- 
ated at  the  point  of  union. 

This  typical  arrangement  of  nerve  roots  and  their  association 
in  the  formation  of  single  pairs  of  metameric  nerves  is  a  con- 
stant one  in  all  vertebrates  and  is  already  suggested  by  the 
somewhat  more  primitive  condition  in  Amphioxus.  In  the 
lower  phylogenetic  stages,  however,  the  plan  is  a  little  less 
precise,  and  there  is  sufficient  indication  to  show  that  here,  too, 
as  elsewhere,  the  final  arrangement  has  been  obtained  by  a 
natural  development  from  a  less  definite  one. 

Thus  in  Amphioxus  the  motor  roots  consist  of  a  series  of 
fibers  distinct  from  one  another;  the  sensory  roots  are  more 
definite  and  are  placed  in  the  intervals  between  the  first,  in 
such  a  manner  that  the  motor  roots  correspond  to  the  myo- 
meres,  the  sensory  roots  to  the  myocommata.  Moreover,  since 
in  this  singular  animal  the  body  somites  on  the  two  sides  do 
not  match  but  alternate  with  one  another,  the  nerve  roots  do 
the  same,  and  a  sensory  root  of  one  side  will  lie  in  the  same 
transverse  plane  as  a  motor  root  of  the  other.  The  sensory 
and  motor  roots  do  not  unite,  and  the  former  becomes  as- 
sociated with  a  subcutaneous  ganglion. 

This  alternate  arrangement  of  the  roots,  excepting  the 
non-correspondence  between  the  two  sides,  is  continued  in  most 
fishes.  In  the  selachians,  for  example,  the  motor  root  passes 
through  a  foramen  in  the  side  of  the  vertebra,  the  sensory  root 
through  a  similar  foramen  in  the  intercalary  piece ;  the  latter  is 
thus  inter-,  the  former  intra-vertebral. 

As  we  ascend  the  series  the  tendency  is  more  and  more 
*  The  sensory  roots  often  contain  motor  fibers. 


THE   NERVOUS    SYSTEM  437 

towards  an  intervertebral  exit  for  both  roots,  but  even  in 
birds  and  mammals  there  are  cases  of  an  exit  through  a  verte- 
bra. Thus  in  the  pre-sacral  vertebrae  of  birds  there  are  two 
foramina  on  each  side  in  the  bodies  of  vertebrae  for  the  separate 
exit  of  the  two  roots;  there  are  also  many  instances  among 
mammals  of  the  piercing  of  a  vertebra  for  nerve  exits;  for 
example,  the  majority  of  the  cervical  and  dorsal  vertebrae  in 
pigs,  or  the  dorsal  and  lumbar  vertebrae  among  ruminants. 

Regarding  the  union  of  dorsal  and  ventral  roots ;  in  the  cy- 
clostome  Petromyzon  the  two  remain  separate,  although  in 
other  cyclostomes,  (e.g.,  Myxine)  they  unite.  In  true  fishes 
the  union  of  the  two  roots  takes  place  outside  of  the  vertebral 
canal ;  in  the  higher  forms  the  union  is  within  it  and  the  united 
nerves  pass  through  the  inter-  (or  intra-)  vertebral  foramen. 
The  spinal  ganglia,  which,  in  the  higher  forms,  are  exclusively 
associated  with  the  sensory  roots,  are  often  in  the  lower  con- 
nected with  both.  They  may  possibly  be  homologous  with  the 
subcutaneous  ganglia  found  on  the  sensory  nerves  of  Amphi- 
v.vus,  but  their  development  from  a  ridge  along  the  spinal  cord 
does  not  seem  to  support  this  idea. 

In  studying  the  distribution  of  the  peripheral  nervous  system 
there  are  two  fundamental  principles  to  be  first  considered,  ( I ) 
that  of  the  exact  relation  between  the  size  of  a  nerve  and  the 
amount  of  development  of  the  part  to  which  it  is  distributed ', 
and  (2)  that  of  the  permanence  of  nerve  distribution.  The 
first  follows  from  the  fact  that  every  cell  or  related  group  of 
cells  in  a  given  organ  has  each  its  own  nerve  fiber,  and  there 
are  thus  as  many  fibers  in  the  nerve  bundle  supplying  the  organ 
as  there  are  such  units  in  the  organ  itself.  If,  then,  a  part  re- 
duces or  increases  its  total  number  of  cells,  the  change  is  di- 
rectly indicated  in  a  corresponding  reduction  or  addition  of 
nerve  fibers;  and,  furthermore,  as  the  separate  nerve  fibers 
must  each  reach  a  central  cell  in  the  brain  or  cord,  there  are 
changes  there  also.  These  latter  are  sometimes  sufficient  to 
become  easily  noticeable,  as  in  the  case  of  the  intumescentias  of 
the  spinal  cord,  which  are  correlated  with  the  development  of 
the  limbs. 


438  HISTORY    OF    THE    HUMAN    BODY 

The  second  principle,  that  of  the  permanence  of  nerve  dis- 
tribution, has  already  been  referred  to  in  several  places,  since 
many  of  our  safest  and  surest  conclusions  concerning  homolo- 
gies  are  based  upon  it.  This  principle,  more  fully  expressed, 
affirms  that  a  part  never  changes  its  nerve  supply,  and  that  a 
given  nerve,  once  associated  with  a  certain  organ  or  complex 
\of  organs,  will  follow  it  through  all  its  subsequent  transforma- 
tions and  even  migrations.  A  good  illustration  of  this  is  seen 
in  the  history  of  the  stapedius  muscle  of  the  middle  ear,  which 
is  supplied  by  a  branch  of  the  facial  nerve.  This  supply  is  by 
no  means  the  most  convenient,  and  is  reached  only  through 
overcoming  a  series  of  mechanical  disadvantages,  yet  it  is 
rendered  necessary  by  the  fact  that  the  muscle  in  question  was 
once  a  part  of  the  digastricus  (the  posterior  belly  of  the  mam- 
malian muscle  of  the  same  name),  and  as  such  was  supplied 
by  the  Facialis.  Through  the  application  of  this  inviolable 
principle  numerous  homologies  have  been  established,  and 
others,  long  believed  in,  have  been  disproven. 

Of  undoubted  connection  with  this  close  correspondence  be- 
tween peripheral  nerves  and  the  organs  to  which  they  are  dis- 
tributed, as  enunciated  in  the  above  principles,  is  the  singular 
phenomenon  of  plexus  formation,  seen  in  the  nerves  which 
supply  the  limbs.  These  plexuses  consist  of  a  more  or  less 
intricate  set  of  intercommunications  between  the  spinal  nerves 
that  are  distributed  to  the  limbs,  and  are  hence  two  in  number, 
plexus  brachialis  and  plexus  lumbo-sacralis,  involving  the 
nerves  which  supply  the  anterior  and  posterior  limbs  respec- 
tively. The  number  of  nerves  involved  in  each  plexus  differs 
considerably,  and  reaches  a  large  number  in  certain  fishes,  in 
which  the  fins  are  associated  with  a  large  number  of  myotomes, 
but  in  animals  with  the  hand  form  of  limb  (chiridia)  the  num- 
ber varies  between  two  and  seven.  Of  this  series  one  or  two, 
usually  the  central  ones,  perform  the  greater  part  of  the  task 
of  supplying  the  limb,  and  are  consequently  the  largest;  the 
others  grade  off  above  and  below  to  those  of  normal  size.  The 
number  and  complexity  of  the  intercommunications  also  reach 


THE   NERVOUS    SYSTEM  439 

their  extremes  in  the  center  of  the  plexus  in  connection  with 
these  larger  nerves,  above  and  below  which  the  nerves  become 
gradually  less  involved,  until  those  are  reached  which  have  so 
slight  a  connection  with  the  plexus  that  they  are  included 
within  it  by  some  authors  and  not  by  others.  There  is,  in  fact, 
considerable  individual  variation  in  a  given  plexus,  and  a 
debatable  nerve  may  furnish  a  communicating  branch  in  one 
specimen  which  may  be  absent  in  another. 

The  organization  of  a  plexus  may  be  best  learned  by  actual 
examples,  for  which  the  brachial  plexuses  of  two  amphibians, 
two  birds,  and  two  mammals  may  be  selected  (Fig.  123). 
From  these  it  will  be  seen  that  not  only  is  the  number  of  nerves 
involved  a  different  one,  but  that  the  nerves  themselves  are 
not  the  same,  counting  from  the  first.  This  latter  fact  is  but 
another  way  of  saying  that  the  girdles  shift  along  the  columns 
in  different  animals,  locating  in  all  cases  at  the  point  where  the 
support  will  be  the  most  effective,  a  fact  brought  out  in 
previous  chapters  in  relation  to  the  bones  and  muscles.  It 
shows  clearly  that  homologies  cannot  rest  upon  definite  body 
metameres,  since  there  is  great  variation,  both  in  the  total  num- 
ber of  metameres  and  in  the  relative  length  of  each  subdivision 
of  the  body;  neck,  trunk  and  tail.  Thus  in  a  frog,  with  a  total 
of  but  ten  pairs  of  spinal  nerves,  the  brachial  plexus  involves 
the  first  three  and  the  lumbo-sacral  plexus  the  last  four,  leaving 
but  three  pairs  of  spinal  nerves  not  involved  in  plexus  forma- 
tion ;  yet  the  metameres  thus  represented  cannot  be  taken,  meta- 
mere  for  metamere,  as  the  homologues  of  the  first  ten  of  other 
animals,  which,  in  some  cases,  as  in  most  birds,  for  example, 
would  be  included  entirely  in  the  neck ;  it  may  rather  be  said 
that  the  ten  of  the  frog  are  homologous  in  a  general  way  with 
the  total  number  of  other  animals,  the  two  plexuses  serving  as 
fixed  points  for  comparison. 

In  comparing  the  various  forms  of  plexus  with  one  another, 
there  are,  in  spite  of  the  great  diversity  of  combinations,  cer- 
tain points  of  similarity.  In  the  first  place,  both  plexuses  are 
alzvays  formed  entirely  of  the  ventral  divisions  of  the  spinal 


440 


HISTORY    OF    THE    HUMAN    BODY 


nerves,  the  dorsal  branches  being  in  all  respects  similar  to  those 
of  adjacent  nerves;  secondly,  the  final  outcome  of  the  branch- 
ing results  in  the  formation  of  distinct  dorsal  and  ventral 


xvn 


XVIII 


jxix 


XX 


FIG.  123.     Brachial  plexus  of  various  vertebrates. 

(a)  Frog.  [After  GAUPP.]  (b)  Axolotl  (urodele).  [After  FURBRINGER.]  (c) 
Cassowary.  [After  FURBRINGER.]  (d)  Domestic  fowl.  [FURBRINGER.]  (e)  Dog. 
[After  ELLENBERGER  and  BA'UM.]  (f)  Man.  [GEGENBAUR.] 

The  roman  numerals  indicate  the  spinal  nerves.  The  other  abbreviations  are 
sufficiently  complete  to  designate  the  nerves  coming  from  the  plexuses. 


THE    NERVOUS    SYSTEM  441 

branches,  each  one  or  two  in  number,  the  former  distributed 
along  the  extensor,  the  latter  along  the  flexor,  aspect.  In  the 
figures  given,  which  are  drawn  from  the  ventral  side,  the  dorsal 
elements  are  represented  as  forming  a  deeper  layer,  and  are 
shaded  for  the  purpose  of  rendering  them  more  distinct.  It 
will  be  seen,  also,  that  each  of  these  final  elements  involves 
more  than  one  root,  and  also  that  the  same  roots  furnish 
fibers  for  more  than  one  nerve.  Furthermore,  owing  to  the 
embryonal  relation  of  the  chiridium  to  the  body,  the  first  digit 
being  anterior  in  both  cases,  the  nerves  supplying  the  inner 
(radial  or  tibial)  side  of  each  limb  are  derived  more  from  the 
anterior  portion  of  the  plexus ;  those  supplying  the  outer  side 
from  the  posterior.  Based  upon  the  principles  given  above, 
the  formation  of  a  plexus  possesses  great  morphological  signifi- 
cance; for  its  intercommunications  and  its  branchings  are, 
in  part  at  least,  records  of  the  past  history  of  the  limbs,  rec- 
ords which  are  so  complicated  that  but  little  progress  has  as 
yet  been  made  in  their  interpretation.  It  may  be  supposed, 
however,  that  if  any  two  parts,  two  muscles,  for  example, 
each  supplied  by  its  own  nerve,  should  coalesce,  their  nerves 
would  also  fuse  into  a  single  bundle,  at  least  distally,  and  even 
that  the  extent  of  this  fusion,  that  is,  the  distance  from  the 
origin  at  which  these  two  nerves  come  together,  would  meas- 
ure the  relative  length  of  time  the  parts  have  remained  fused. 
Similarly,  if  a  single  part  should  differentiate  into  two,  a 
phenomenon  constantly  occurring  among  limb  muscles,  the 
nerve  would  branch;  and,  furthermore,  the  increase  of  the 
differentiation  between  them,  that  is,  a  gain  in  the  independ- 
ence of  action,  would  tend  to  separate  the  nerve  still  more 
and  cause  the  point  of  bifurcation  to  move  proximally. 

It  is  thus  probable  that  the  plexuses  have  a  meaning  for 
him  who  is  able  to  read  it,  the  well-known  conservatism 
of  nerves  in  regard  to  their  course  assisting  greatly  in  the 
preservation  of  these  records.  This  conservatism  is  well  shown 
in  the  case  of  snakes,  in  which  the  limbs  have  been  lost,  but 
where  there  are  still  traces  of  the  plexuses,  a  fact  attesting  the 
former  presence  of  the  limbs.  In  certain  other  cases,  as  in  the 


442  HISTORY    OF    THE    HUMAN    BODY 

Gymnophiona  and  in  the  lost  hind  limbs  of  the  urodele  Siren, 
not  only  have  the  limbs  and  their  girdles  utterly  vanished,  but 
there  is  also  no  trace  of  a  plexus,  showing  that  since  the  reduc- 
tion of  the  limbs  a  much  longer  time  has  elapsed  than  in  the 
former  case,  a  conclusion  in  full  accord  with  the  relative  place 
of  these  animals  in  the  system  and  in  their  geological  appear- 
ance. 

It  is  not  probable,  however,  that  all  the  changes  in  a  plexus 
have  a  historic  significance,  since  another  factor  must  be  taken 
into  consideration,  one  that  is  the  cause  of  certain  changes, 
especially  those  of  an  individual  character.  This  factor  is 
found  in  the  evident  tendency,  of  certain  forms  at  least,  to 
shift  the  position  of  their  girdles.  This  tendency  is  shown  in 
individual  cases  by  an  increase  in  the  size  of  certain  of  the 
nerves  involved  and  a  corresponding  diminution  in  that  of  those 
either  anterior  or  posterior  to  them ;  and,  in  certain  species,  by 
making  careful  counts  of  the  separate  fibers  of  the  main  nerves 
in  a  large  number  of  individuals  of  a  given  species,  the  direc- 
tion in  which  the  girdle  is  migrating  has  been  definitely  estab- 
lished. Thus  in  the  common  toad  there  is  shown  a  tendency 
to  push  the  shoulder  girdle  still  further  anteriorly,  and  as  its 
present  position  is  extremely  cephalic,  the  continued  tendency 
must  be  an  instance  of  the  inertia  of  variation  through  which 
a  line  of  development,  once  started,  is  often  carried  far  beyond 
the  point  of  greatest  efficiency.  This  procedure  involves  more 
generally  the  posterior  than  the  anterior  girdle,  and  hence  the 
lumbo-sacral  plexus  is  more  apt  to  vary  individually.  This 
migratory  tendency  may  result  in  the  establishment  in  a  given 
species  of  two  or  three  types  of  plexus,  to  which  all  individual 
variations  may  be  referred,  as  has  been  established  in  the  case 
of  the  urodele  Necturus,  well  known  also  for  its  variability  in 
pelvic  attachment. 

The  early  anatomists,  by  a  careful  count  of  the  nerve  roots 
as  they  were  found  proceeding  from  the  brain  in  the  human 
subject,  enumerated  the  following  twelve  pairs  of  cranial 
nerves,  that  is,  of  nerves  which  originate  within  the  cranial 
cavity  and  escape  through  foramina  in  the  bone: 


THE    NERVOUS    SYSTEM  443 

NAME  FUNCTION 

I.  Olfactorius  special  sense,  smell. 

II.  Opticus  special  .sense,  sight. 

III.  Motor  oculi  motor. 

IV.  Trochlcaris  [Patheticus]  motor. 

V.  Trigetmnus  [Trifacial]       {  mainly  sensory,  with  a  small  motor 

(          root 

VI.  Abducens  motor. 

VII.  Facialis  mixed. 

VIII.  Acusticus   [Auditprius]  special  sense,  hearing. 

IX.  Glosso-pharyngeus  mixed. 

X.  Vagus  [Pheumogastricus]  mixed. 

XI.  Accessorius   [Willisii]  mixed,  mainly  motor. 

XII.  Hypoglossus  mixed. 

Of  these  the  first  two  arise  from  the  primary  fore-brain, 
the  tel-  and  di-encephalon  respectively ;  the  remaining  ten  take 
their  origin  from  the  met-  and  myelencephala,  leaving  the 
mesencephalon  without  any.  It  would  thus  seem  that  the 
former  may  be  nerves  of  the  archencephalon  or  primary  brain, 
laid  down  in  Amphioxus,  while  the  latter  belong  to  the  second- 
ary addition  from  the  anterior  end  of  the  original  spinal 
cord.  The  last  ten  were  thus  at  first  spinal  nerves,  in  which, 
in  spite  of  their  extreme  specialization,  it  might  be  possible  to 
recognize  the  original  spinal  elements,  each  with  its  sensory 
and  motor  roots,  its  accompanying  ganglion,  and  so  on.  That 
the  original  elements  have  in  some  cases  become  modified  is 
evidenced  by  several  facts,  first,  the  existence  among  them  of 
wholly  motor  nerves  without  sensory  fibers  and  lacking  a  gan- 
glion ;  and,  again,  the  fact  that  some  of  the  nerves  in  the  above 
list  are  shown  to  be  composed  of  several  primary  nerves  by 
their  origin  from  multiple  roots,  or  from  the  presence  of  sev- 
eral associated  ganglia.  The  twelfth  nerve  is  outside  of  the 
cranium  in  fishes,  and  becomes  later  included  within  it,  proba- 
bly by  the  fusion  with  the  skull  of  the  vertebra  with  which  it 
is  associated.  The  eleventh  is  closely  associated  with  the 
Vagus  and  appears  as  a  distinct  cranial  nerve  only  in 
mammals. 

Aside  from  the  elements  found  in  the  above  there  are  traces 


444  HISTORY    OF    THE    HUMAN    BODY 

of  several  other  spinal  elements  originally  belonging  to  the 
primary  anterior  end  of  the  cord,  which  do  not  survive  in 
the  higher  forms  as  definite  cranial  nerves.  These  are  desig- 
nated as  the  spino-occipital  nerves,  and  are  first  met  with  in 
the  selachians,  where  they  appear  as  1-5  pairs,  placed  very 
far  back,  along  the  medulla.  They  are  spinal  in  character 
and  not  associated  with  the  other  cranial  nerves,  although 
they  are  all  included  within  the  skull.  As  this  latter  part 
ends  abruptly  with  the  otic  region  in  cyclostomes,  and  is  im- 
mediately followed  by  the  successive  pairs  of  true  spinal 
nerves,  it  seems  reasonable  to  suppose  that  when,  in  the  sela- 
chians, the  cranial  cavity  became  enlarged  by  an  addition  at 
the  posterior  end,  several  of  the  original  spinal  nerves  were 
included,  forming  the  nerves  in  question.  In  the  higher  car- 
tilaginous fish  (Holocephali),  and  in  ganoids,  this  set  of 
nerves  becomes  reduced  to  two  pairs,  yet  a  second  set,  also 
of  1-5  pairs,  has  been  taken  in,  presumably  in  the  same  way. 
To  distinguish  between  these  two  sets  of  spino-occipital  nerves, 
the  first  are  termed  occipital,  the  second  occipito-spinal.  Rep- 
resentatives of  both  sets  occur  in  varying  proportions  in  other 
fishes,  but  in  the  amphibians  they  seem  to  have  wholly 
disappeared,  and  are  never  seen  again  as  distinct  nerves.  Al- 
though nothing  has  as  yet  been  definitely  proven  in  the  matter, 
it  is  probable  from  other  evidence  that  above  the  fish  the 
occipital  region  suffers  considerable  reduction,  during  which 
many  of  these  elements  may  have  become  lost,  while  others 
may  have  become  established  among  the  root  elements  of  the 
twelfth  nerve,  the  hypoglossal,  since  this  nerve  appears  first 
as  a  cranial  element  in  the  reptiles  and  continues  throughout 
Sauropsida  and  Mammalia. 

For  purposes  of  description  and  with  reference  to  their 
morphology  the  cranial  nerves  fall  naturally  into  groups  which 
are  best  considered  separately.  These  may  now  be  taken  up 
in  detail. 

i.  THE  ANTERIOR  GROUP.    (Qlfactorius  and  Options.) 
These,  the  two  first  in  the  list,  are  nerves  of  special  sense, 
the  fibers  of  which  are  distributed  respectively  to  the  nasal- 


THE   NERVOUS    SYSTEM  445 

mucous  membrane  and  to  the  retina.  The  morphological 
position  is  doubtful,  for,  while  they  are  considered  by  some 
to  be  the  first  true  cranial  nerves  and  to  belong  to  a  much 
earlier  period  than  any  of  the  rest,  others  deny  them  the  right 
to  be  called  nerves  at  all,  and  treat  them  as  parts  of  the  brain, 
the  olfactory  lobes  (rhinencephalon)  and  the  optic  stalks  re- 
spectively. 

Attempts  have  been  made  to  bring  this  condition  into  ac- 
cord with  that  found  in  Amphioxus,  for  here  the  archen- 
cephalon,  a  rudimentary  brain  formed  by  the  enlargement  of 
the  anterior  end  of  the  spinal  cord  and  possibly  the  equiva- 
lent of  the  telencephalon  of  vertebrates,  bears  two  rudimentary 
sense-organs,  the  first  an  olfactory  pit  and  the  second  a  pigment 
speck.  Of  these  the  first  is  connected  with  the  brain  by  a  short 
diverticulum,  while  the  second  is  embedded  within  the  brain 
wall.  Although  similarity  of  function  of  the  two  sets  of  organs 
in  the  two  cases  tempts  one  to  believe  in  an  homology  between 
them,  the  decision  really  hinges  upon  the  identity  of  these 
sense-organ  rudiments  and  the  perfected  organs  of  the  higher 
vertebrates ;  for  if  the  olfactory  groove  and  the  pigment  speck 
are  historically  the  anlagen  of  the  nose  and  eye,  a  point  not 
definitely  established,  then  the  identity  of  the  nerves  with 
the  corresponding  parts  of  the  archencephalon  naturally  fol- 
lows. In  favor  of  this  latter  assumption  is  the  fact  of  the 
origin  of  these  tzvo  nerves  from  the  primary  fore-brain,  while 
none  of  the  others  arise  anterior  to  the  metencephalon.  An 
entirely  problematical  element  belonging  to  this  region  is  that 
of  a  definite  pair  of  nerves,  Nervus  terminalis,  which  occur  in 
all  selachians,  and  extend  from  the  anterior  part  of  the  telence- 
phalic  lobes,  where  they  originate,  along  the  anterior  aspect 
of  the  olfactory  stalks.  As  they  have  been  but  recently  dis- 
covered they  have  escaped  enumeration  with  the  classical 
twelve  pairs;  their  origin  from  telencephalon  is  also  anoma- 
lous. Nothing  can  as  yet  be  predicted  of  their  morphological 
significance. 

II.    THE    MOTOR    NERVES    OF   THE   EYEBALL.        (Motor    OCllH, 

Trochlearis  and  Abducens.) 


446 


HISTORY   OF   THE    HUMAN    BODY 


These  three  pairs  are  small  and  very  special  nerves,  having 
no  other  distribution  than  the  six  muscles  of  the  eyeball. 
They  are  thus  exclusively  motor,  and  on  the  theory  that  the 
cranial  nerves,  excepting,  perhaps,  those  of  the  preceding 
group,  represent  modified  spinal  nerves,  seem  to  correspond 
to  the  ventral  roots  of  three  original  nerves,  the  sensory  roots 
of  which  are  either  lost,  or,  more  probably,  contained  in  the 

,**m 


a 


Mes 


FIG.  124.     The  Nervus  terminalis  of  the  selachians.     [After  LOCY.] 

(a)  Dorsal  view  of  the  brain  of  the  dog-fish,  Squalus  acanthias.  (b)  Horizontal 
section  through  the  anterior  part  of  the  same,  showing  the  origin  of  Nervus  ter- 
minalis. 

Nas,  nasal  capsule;  Olf ',  olfactory  lobe;  N.  ter,  Nervus  terminalis;  gl,  its  ganglion; 
Tel.,  telencephalon;  Di,  diencephalon;  Mes,  mesencephalon;  Met,  metencephalon; 
Myel,  myelencephalon. 

sensory  elements  of  adjacent  cranial  nerves  such  as  Trige- 
minus  or  Facialis.  The  fact  which  suggests  this  hypothesis 
most  strongly  is  the  strictly  metameric  character  of  their 
field  of  distribution,  namely,  the  eye  muscles  themselves,  as 
is  shown  by  their  developmental  history.  In  selachian  em- 
bryos, which  have  preserved  this  early  history  more  completely 
than  have  the  higher  forms,  there  develops  in  the  head  a  series 
of  myotomes,  similar  to  and  continuous  with  those  of  the 
trunk.  Some  of  them  soon  atrophy,  but  the  first  three  fold 


THE    NERVOUS    SYSTEM 


447 


about  the  developing  eyeball  and  furnish  it  with  muscles. 
From  the  first  arise  three  of  the  straight  muscles  and  one 
oblique,  from  the  second  the  other  oblique,  and  from  the 
third  the  remaining  straight  muscle.  These  three  myotomes 
are  innerved  by  the  three  nerves  under  consideration,  and  in 
their  natural  order  of  succession,  as  follows : 


SOMITE 

MUSCLES   DEVELOPED 

NERVE 

No. 

Myotome  I 

Rectus  superior 
Rectus  intemus  A1** 
Rectus  inferior   IT 
Obliquus  inferior 

Motor  oculi 

Ill 
IV 

Myotome  II 

Obliquus  superior 

Trochlearis 

Myotome  III 

Rectus  externus  fif  fl 

Abducens 

VI 

These  relationships  are  constant  throughout  all  vertebrates, 
corroborating  the  idea  that  we  have  here  the  enumeration  of 
some  very  primitive  morphology.  In  certain  Orders,  in  re- 
sponse to  special  needs,  other  special  muscles  appear  in  con- 
nection with  the  eyeball,  but  these  are  seen  to  be  differentia- 
tions of  certain  of  the  above,  and  retain  the  same  innervation ; 
thus  the  retractor  bulbi  *  arises  from  the  external  rectus,  and, 
like  it,  is  innerved  by  the  sixth  nerve. 

The  relation  of  these  three  nerves  to  adjacent  sensory  ele- 
ments and  their  right  to  be  considered  ventral  roots  are  matters 
concerning  which,  although  much  has  been  done,  few  definite 
conclusions  may  be  drawn  as  yet.  The  Motor  oculi,  although 
its  fibers  are  purely  motor,  yet  becomes  connected  with  the 
small  ciliary  ganglion,  through  which  its  fibers  innerve  the 
ciliary  muscles  and  the  iris.  This  ganglion  may  have  the 
morphological  value  of  the  one  belonging  to  a  sensory  root 
now  lost,  a  conclusion  which  would  make  this  nerve  an  entire 
spinal  element  with  a  reduction  of  the  sensory  root.  Other 
views  associate  with  it  as  its  sensory  element  a  portion  of  the 
Trigeminus.  The  Trochlearis,  although  essentially  a  motor 
nerve,  possesses  in  fishes  and  amphibians  a  few  sensory  fibers, 

*  This  muscle  is  rudimentary  or  wanting  in  the  Anthropoidea. 


448  HISTORY    OF    THE    HUMAN    BODY 

yet,  in  spite  of  this,  all  are  agreed  that  the  sensory  element 
originally  associated  with  this  has  become  incorporated  with 
the  Trigeminus.  The  sensory  portion  of  the  Abducens  is 
probably  also  a  part  of  the  Trigeminus,  although  certain 
facts  indicate  an  association  with  the  Facialis. 

III.  THE  TRIGEMINUS-FACIALIS  GROUP.     ( Trf^eminus'  Fa- 
cialis, Acusticus.) 

This  group  and  the  next  are  by  far  the  most  extensive,  and 
together  constitute  the  main  bulk  of  the  nerves  of  the  head. 
Their  relationships  differ  considerably  in  fishes  and  aquatic 
amphibians  on  the  one  hand  [Plate  VI],  and  in  terrestrial 
(and  secondarily  aquatic)  vertebrates  on  the  other  [Plate 
VII],  a  difference  largely  due  to  the  presence  in  the  one  and 
the  suppression  in  the  other  of  an  extensive  system  of  ex- 
ternal sense-organs  of  uncertain  function  but  undoubtedly 
of  assistance  in  an  aquatic  life.  These  organs,  variously 
termed  "  integumental  sense-organs "  or  "  dermal  canal  sys- 
tem'' are  visible  externally  and  are  arranged  in  definite  lines 
running  about  the  head  and  continued  in  a  single  (or  double) 
longitudinal  row,  the  lateral  line,  down  the  sides  of  the  body. 
The  system  of  nerves  which  supplies  these  is  shown  in  the 
first  of  the  accompanying  diagrams,  and  consists  of  three  su- 
perficial trunks  directed  forwards  and  a  fourth  one  directed 
backwards,  the  former  referred  to  the  Facialis  (VII),  the  lat- 
ter to  the  Vagus  (X).  To  each  trunk  there  belongs  typi- 
cally a  special  ganglionic  swelling,  placed  near  its  origin ;  but 
these  are  distinct  in  only  a  few  forms  (selachians,  dipnoans,  a 
few  aquatic  amphibians)  and  in  all  others  become  completely 
fused  with  the  ganglion  semilunare  of  the  Trigeminus  and 
are  demonstrable  only  in  the  embryo.  To  the  compound  gan- 
glion thus  resulting,  which  is  found  in  most  amphibians  and 
in  all  the  amniotes,  may  be  applied  the  term  Gasserian,  long  in 
use  in  human  anatomy  for  this  organ. 

The  most  dorsal  of  the  three  Facialis  branches  of  this  sys- 
tem is  the  superficial  ophthalmic  (ramus  ophthalmiais  super- 
ficialis  Septimi),  and  is  accompanied  by  a  like-named  branch 
of  the  Trigeminus  (ramus  ophthalmiais  superficialis  Quinti), 


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THE   NERVOUS    SYSTEM  449 

which  supplies  general  sensation  to  this  region.  The  second 
branch  is  the  buccal  (ramus  buccalis),  accompanied  in  its  turn 
by  the  maxillary  branch  of  the  Trigeminus.  The  third  is 
the  external  mandibular,  divided  into  anterior  and  posterior 
branches.  The  companion  branch  from  the  Trigeminus, 
associated  with  the  anterior  of  the  two  subdivisions,  is  the 
mandibular  of  that  nerve  (ramus  mandibularis  Quinti). 
In  the  Dipnoi  there  is  a  communicating  branch  between  this 
part  of  the  system  and  that  belonging  to  the  Vagus,  but  this 
seems  to  be  wanting  in  other  cases. 

The  remaining  branches  of  the  facial  nerve  are  divisible  into 
two  portions,  sensory  and  motor.  The  sensory  portion  pos- 
sesses at  its  origin  the  large  genicular  ganglion,  from  which 
proceed  (i)  a  large  palatine  branch  and  (2)  a  small  internal 
mandibular.  The  motor  portion  incorporates  within  itself  the 
external  mandibular  branch  of  the  lateral  line  system  given 
above,  and  this  forms  the  mixed  hyo-mandibular  branch  which 
supplies  the  region  of  the  lower  jaw  and  the  hyoid  arch. 

Much  of  the  Trigeminus  has  already  been  described  in  as- 
sociation with  the  Facialis.  There  remain  to  be  mentioned 
the  large  semilunar  ganglion  (often  fused  with  the  sensory 
ganglia  of  the  lateral  line  nerves  of  Facialis)  which  lies  at 
the  base  of  the  three  branches  already  described,  and  the 
deep  ophthalmic  branch  (ramus  ophthalmicus  profundus). 
This  latter  possesses  a  ganglion  of  its  own  and  issues  from  the 
skull  by  a  separate  foramen.  It  is  thus  semi-distinct  from  the 
remainder  and  may  be  considered  a  separate  nerve,  originally 
anterior  to  the  Trigeminus,  and  secondarily  associated  with 
it.  There  are  some  indications  to  show  that  it  may  have 
once  been  associated  with  the  Trochlearis,  as  sensory  and 
motor  roots,  respectively,  of  the  same  elementary  nerve.  The 
mandibular  branch  of  the  Trigeminus  possesses  a  few  motor 
elements,  which  prevent  the  nerve  from  being  classed  as 
wholly  sensory. 

The  later  history  of  the  parts  above  considered  may  be  fol- 
lowed from  the  second  diagram  [Plate  VII],  which  represents 
in  a  general  way  the  terrestrial  type,  but  which  in  its  propor- 


450  HISTORY   OF   THE    HUMAN    BODY 

tions  and  certain  other  details  suggests  more  especially  the 
mammalian  condition.  The  first  and  most  striking  change  is 
the  loss  of  the  special  system  supplying  the  lateral  line  sense- 
organs,  the  entire  equipment  for  which  vanishes  in  amphibians 
that  become  terrestrial  and  never  reappears.  This  causes  at 
jfonce,  among  other  changes  to  be  considered  later  on,  a  loss 
of  three  branches  of  the  facial  nerve,  the  superficial  ophthalmic, 
the  buccal,  and  the  external  mandibular.  There  remain  the 
palatine,  the  internal  mandibular  and  the  motor  element  of 
the  hyo-mandibular,  of  which  the  first  two  become  reduced  in 
size  and  fuse  with  Trigeminus  elements,  while  the  third  loses 
in  one  direction  but  more  than  compensates  for  it  in  another. 
The  palatine,  under  the  name  of  N.  petrosus  superficialis  major, 
passes  through  the  pterygoid  \_Vidian~}  canal  (in  Mammals) 
and  enters  the  spheno-palatine  ganglion  of  the  sympathetic 
system,  where  it  meets  with  fibers  of  the  Fifth  nerve,  and  con- 
tinues under  the  name  of  palatinus  major,  usually  considered  as 
a  part  of  the  Trigeminus.  The  internal  mandibular,  now 
known  as  the  chorda  tympani,  transverses  the  tympanic  cavity, 
running  between  malleus  and  incus.  It  leaves  the  middle  ear 
through  the  Glasserian  fissure  (in  mammals)  and  blends  with 
the  lingual  branch  of  ramus  mandibularis  V.,  encountering  in 
its  course  the  sub-maxillary  ganglion  (better,  sub-mandibular) 
of  the  sympathetic  system. 

The  explanation  of  these  complicated  relationships  becomes 
clear  when  we  consider  the  history  of  the  related  parts.  Mal- 
leus and  incus  are  primarily  the  condyle  of  the  jaw  and  the 
quadrate  bone,  respectively,  and  the  branch  in  question  runs 
along  the  articulation  between  them.  In  the  Mammalia  these 
osseous  elements  become  drawn  within  the  cavity  of  the 
middle  ear,  where  they  undergo  a  transformation  into  auditory 
ossicles;  and  the  nerve,  in  order  to  preserve  its  original  rela- 
tionships, must  follow  them,  thus  producing  a  complicated 
condition,  easily  explained  by  their  morphological  history,  but 
wholly  inexplicable  otherwise.  Furthermore,  with  the  in- 
creased importance  of  the  tongue,  and  more  especially  with 
the  development  of  the  fleshy  part  of  it  in  mammals,  the  orig- 


THE    NERVOUS    SYSTEM  451 

inal  branch  of  the  Trigeminus  gains  in  importance  and  the 
two  become  secondarily  associated. 

With  the  reduction  in  bulk  of  the  hyo-branchial  musculature 
the  motor  element  of  the  hyo-mandibular,  the  only  portion 
now  remaining,  tends  to  decrease  in  size,  but  this  is  more  than 
compensated  for  in  mammals  by  the  development  of  the  mi- 
metic muscles.  These  have  been  shown  to  originate  from  the 
integumental  muscular  layer  of  the  neck  region,  innerved  by 
the  branch  under  consideration,  and,  as  this  layer  spreads  up 
over  the  neck  and  differentiates  into  specialized  slips,  the  in- 
nervation  increases  also  and  spreads  eventually  over  the  entire 
face,  thus  gaining  its  right  to  the  name  "  Facialis,"  a  right 
which,  curiously  enough,  it  possessed  originally,  in  connection 
with  the  lateral  line  organs,  but  which  it  afterward  lost  until 
it  regained  through  its  motor  elements  what  it  had  lost  in 
its  sensory.  The  branch  to  the  stapedius  muscle  of  the  middle  .< 
ear,  N.  stapedialis,  proceeds  also  from  the  hyo-mandibular, 
and  comes  originally  from  the  branch  supplying  the  digastric 
muscle. 

The  Trigeminus  of  the  second,  or  terrestrial,  type,  suffers 
no  reduction  through  the  loss  of  the  lateral  line  organs,  since 
it  has  nothing  directly  to  do  with  them,  but  the  four  original 
branches  become  reduced  to  three  through  the  loss  of  the  deep 
ophthalmic  element,  which  seems,  in  part  at  least,  to  fuse  with 
the  superficial  branch  of  the  same  nerve  to  form  the  "  first 
branch  "  of  human  anatomy,  the  ophthalmicus.  The  maxil- 
laris  and  mandibularis  show  but  little  change  and  form  the 
second  and  third  branches,  respectively,  thus  giving  the  reason 
for  the  name  "Trigeminus"  first  applied  in  Man. 

The  great  increase  in  the  size  of  the  lingual  branch  of  the 
mandibularis  has  already  been  noticed ;  otherwise  the  most  im- 
portant innovation  is  found  in  the  new  relations  of  the  Tri- 
geminus with  the  Facialis,  the  Glosso-pharyngeus,  and  the 
sympathetic  ganglia.  The  first  of  these  has  already  been 
treated  in  detail.  The  Glosso-pharyngeus  sends  a  communi- 
cating branch  (tympanic  [Jacobson's~\  nerve)  to  the  otic  gan- 
glion, which  rests  upon  the  base  of  ramus  mandibularis;  the 


452  HISTORY    OF    THE    HUMAN    BODY 

two  nerves  also  come  into  indirect  contact  in  the  tongue,  where 
the  fibers  of  the  gustatory  and  lingual  branches  of  the  two 
nerves  interlace.  Four  ganglia  of  the  sympathetic  system,  the 
entire  cephalic  group,  become  associated  with  the  Trigeminus, 
the  ciliary  with  the  first  branch,  the  spheno-palatine  with  the 
second,  and  the  otic  and  submaxillary  with  the  third. 

The  Acusticus  (eighth  nerve)  is  originally  a  part  of  that 
Facialis  element  which  supplies  the  lateral  line,  and  as  the 
essential  part  of  the  ear,  the  labyrinth,  closely  resembles  in 
its  early  development  the  sense-organs  of  the  lateral  line,  the 
suggestion  is  strongly  felt  that  we  have  here  a  case  of  the 
local  specialization  of  a  single  element  out  of  a  series  of  simi- 
lar parts,  and  that  the  Eighth  nerve  is  consequently  nothing 
more  than  a  branch  of  the  superficial  sensory  system  of  the 
Facialis,  the  region  of  distribution  of  which  chanced  to  develop 
a  high  degree  of  complexity  as  an  organ  of  special  sense. 

IV.  THE  VAGUS  GROUP.  (Glosso-pharyngeus,  Vagus,  Ac- 
cessorius. ) 

This  group  includes  in  all  vertebrates  the  Glosso-pharyngeus 
and  Vagus  nerves,  to  which  is  added  in  mammals  the  Acces- 
sorius,  secondarily  derived  from  the  Vagus,  and  existing  in 
the  Sauropsida  as  a  semi-independent  slip.  This  group  is  pri- 
marily associated  with  the  gill-region,  but  secondary  sends 
branches  backwards  and  forwards  which  may  even  reach  the 
extreme  ends  of  the  body,  thus  having  a  more  extensive  dis- 
tribution than  that  of  any  other  cranial  nerves.  In  the  lower 
forms  this  group  is  extremely  regular  and  possesses  a  well- 
pronounced  metamerism,  thus  strongly  suggesting  its  origin 
from  spinal  nerves,  similar  to  those  which  form  a  direct  con- 
tinuation of  the  series.  Taken  in  connection  with  the  much 
greater  differentiation  of  the  other  cranial  nerves  it  seems 
evident  that  the  acquisition  of  the  anterior  end  of  the  primor- 
dial spinal  cord  by  the  cranium  has  been  a  gradual  one,  and 
that  the  Vagus  group  is  less  modified  than  the  nerves  anterior 
to  it,  because  it  has  been  annexed  later.  The  primitive  condi- 
tion, that  found  in  fishes,  may  be  first  considered  by  the  help 
of  the  diagram  previously  referred  to  [Plate  VI].  The  most 


THE    NERVOUS    SYSTEM  453 

superficial,  and  at  the  same  time,  the  most  aberrant,  is  the  ra- 
mus lateralis,  which  belongs  to  the  system  of  lateral  line  nerves 
and  supplies  the  lateral  line  itself,  which  extends  typically 
to  the  end  of  the  tail.  It  is  thus  the  longest  nerve  in  the  body, 
co-extensive  with  the  spinal  cord  itself.  The  lateral  nerves  of 
the  two  sides  are  connected  with  one  another  by  the  supra-tem- 
poral branch,  which  forms  a  connecting  loop  over  the  top  of  the 
head ;  in  the  Dipnoi,  though  not  in  other  fish,  lateral  communi- 
cating branches  connect  it  with  the  superficial  ophthalmic  nerve 
of  the  Facialis,  thus  uniting  the  two  parts  of  the  system.  The 
ramus  lateralis  has  at  its  proximal  end  a  ganglion  of  its  own 
(ganglion  laterale),  although  it  arises  in  connection  with  the 
combined  ganglionic  mass  of  the  Vagus.  In  spite  of  this  asso- 
ciation, however,  it  is  probable  that  the  ramus  lateralis  did  not 
originally  belong  to  the  Vagus  alone,  but  zvas  built  up  as  a 
collecting  trunk  from  branches  supplied  by  each  metameric 
nerve  of  the  body,  beginning  with  the  Vagus.  The  gradual  \ 
loss  of  these  metameric  connections,  beginning  posteriorly, 
would,  in  time,  leave  the  most  anterior  one  alonef  the  condition 
found  at  present. 

Ventral  to  the  ramus  lateralis  appear  five  elements,  similar 
to  one  another,  each  associated  with  a  gill-slit,  and  possessed  of 
its  own  ganglion.  This  extremely  primitive  condition  is  seen 
in  a  few  forms  only  (e.g.,  the  rays  and  skates),  but  these  ani- 
mals are  in  other  respects  so  primitive,  and  the  condition  is 
so  exactly  what  one  would  expect  as  an  early  one,  that  it  may 
be  taken  as  undoubtedly  the  starting  point.  The  first  of  these 
elements  is  more  distinct  than  the  others,  and  forms  the  Glosso- 
pharyngeus,  treated  as  a  separate  nerve;  the  remaining  four 
are  Vagus  elements  and  in  all  but  very  primitive  forms  arise 
from  a  single  ganglionic  mass,  the  ganglion  jugidare,  formed 
of  a  fusion  of  the  four  primary  ones.  It  is  with  this  that  the 
ganglion  laterale  of  the  ramus  lateralis  is  associated.  Each 
of  these  five  elements  ( Glossus-pharyngeus,  and  the  four  Vagi) 
possesses  an  identical  distribution.  From  the  ganglion  the 
main  stem  passes  downwards,  and  forks  into  two  branches, 
including  a  gill-slit  in  the  fork.  The  two  branches,  one  in 


454  HISTORY   OF   THE    HUMAN    BODY 

front  and  one  behind  the  slit,  are  known  respectively  as  rami 
prcz-  and  post-trematici.  In  this  connection  it  is  interesting  to 
see  that  the  spiracular  opening,  which  probably  represents  the 
gill-slit  next  in  order  anteriorly  to  the  regular  series,  is  simi- 
larly included  between  the  internal  mandibular  (chorda  tym- 
pani)  and  the  hyo-mandibular  of  the  Facialis,  which  thus  be- 
come, respectively,  the  prcz-  and  post-trematic  branches  of  that 
nerve.  It  is  even  possible  in  like  manner  to  consider  the  maxil- 
lary and  mandibular  branches  of  the  Trigeminus  as  similarly 
related  to  the  mouth  opening,  resting  upon  the  probability  of 
the  identification  of  the  mouth  with  an  original  gill-slit  anterior 
to  the  spiraculum.  We  have  thus  a  character  of  great  value 
in  the  resolution  of  the  cranial  nerves  into  their  original  meta- 
meric  elements,  one  which  will  be  considered  later  in  the  treat- 
ment of  this  difficult  and  unsolved  problem. 

Of  these  five  branchial  nerves,  the  Glosso-pharyngeus,  as 
the  most  anterior  and  consequently  the  most  modified  (earliest 
absorbed  by  the  cranium)  possesses  additional  branches  not 
represented  in  the  others,  and  these  have  run  forward  and 
supply  parts  anterior  to  it.  One  of  these  is  a  communicating 
branch  between  this  nerve  and  the  Facialis  and  passes  from  its 
ganglion  (ganglion  petrosum)  to  that  of  the  Facialis  (ganglion 
geniculare).  This  nerve  possesses  no  special  name  in  lower 
forms,  other  than  the  generic  ramus  communicans,  but  in  the 
higher  forms  it  becomes  the  tympanic  (nerve  of  Jacobson), 
and  forms  an  intimate  means  of  communication  between  these 
nerves  and  the  Trigeminus.  Below  this  is  the  palatine,  lying 
near  the  Facialis  branch  of  the  same  name,  and  developing  a 
few  connections  with  it.  A  small  lingual  branch  is  present  in 
the  Dipnoi. 

There  remains  but  one  further  element  to  be  considered, 
but  this  is  an  extensive  one,  the  ramus  intestinalis  Vagi.  This 
appears  in  primitive  forms  as  a  separate  element,  with  its 
own  ganglion,  but  in  all  other  cases  it  arises  with  the  rest  of 
the  Vagus  and  its  ganglion  becomes  lost  in  the  general  mass, 
the  ganglion  jugulare.  This  is  the  branch  which,  even  more 
than  the  lateralis,  has  earned  for  the  nerve  to  which  it  belongs 


THE    NERVOUS    SYSTEM  455 

the  title  of  Vagus  (wandering),  since  it  becomes  distributed 
to  the  oesophagus  and  stomach,  the  heart,  and,  in  higher  forms, 
the  lungs.  In  spite  of  its  great  length  and  extensive  distribu- 
tion, however,  it  is  not  to  be  considered,,  like  the  ramus  lateralis, 
a  compound  nerve,  but  its  length  is  due  rather  to  the  extension 
posteriorly  of  parts  once  placed  far  forward  and  thus  within 
the  legitimate  province  of  the  original  nerve.  Thus,  the  heart 
has  primarily  a  very  anterior  position ;  the  oesophagus  and 
stomach  were  probably  once  very  far  forward,  and  the  lungs 
are  diverticula  of  the  primary  oesophagus.  The  wide  distribu- 
tion of  this  branch  is  thus  a  striking  illustration  of  the  prin- 
ciple of  conservatism  of  nerve  distribution  enunciated  above, 
and  belongs  in  the  same  category  as  the  case  of  the  stapedial 
nerve  or  that  of  the  innervation  of  the  mimetic  musculature. 

In  the  transition  to  terrestrial  life  the  Vagus  group  suffers 
naturally  the  loss  of  the  branchial  elements  in  which  metamer- 
ism zvas  so  clearly  displayed,  but  has  gained  by  the  greater  de- 
velopment of  the  tongue  and  the  sense  of  taste,  and  has  differ- 
entiated the  Accessorius  element.  The  intestinal  branch  alsof 
corresponding  to  the  higher  development  of  its  field  of  distribu- 
tion, is  still  more  extensive  and  complex. 

The  extreme  differentiation  of  this  group  may  be  learned 
from  Plate  VII,  which  here  especially,  in  the  separation  of  the 
Accessorius  and  in  other  points,  suggests  the  mammalian  con- 
ditions. The  communicating  branch,  the  tympanic  nerve,  be- 
comes somewhat  more  complicated  and  forms  connections  be- 
tween four  ganglia ;  starting  from  the  ganglion  petrosum  of  the 
Ninth  nerve  it  runs  forward  and  sends  branches  to  the  gan- 
glion geniculare  of  the  Seventh,  and  to  the  otic  and  sphenopala- 
tine  ganglia  of  the  sympathetic  system.  It  is  also  involved  in  a 
small  plexus  of  sympathetic  nerves  which  surround  the  carotid 
artery.  The  relationships  of  this  nerve,  which  are  so  complex 
in  mammals,  become  especially  so  in  Man,  owing  to  the  short- 
ening of  the  longitudinal  axis  of  the  skull  and  the  formation 
of  the  cervical  flexure,  both  of  which  tend  to  the  shortening 
of  the  distance  between  the  nerve  roots.  The  relations 
found  in  Man  are  shown  in  the  accompanying  figure  (Plate 


456 


HISTORY   OF    THE    HUMAN    BODY 


VIII),  which  is  to  be  carefully  compared  with  Plate  VII. 
As  special  names  are  often  given  in  human  anatomy  to  parts 
spoken  of  by  the  morphologists  under  more  general  terms,  a 
list  of  equivalent  terms  is  here  added,  for  the  better  compari- 
son of  the  figures  alluded  to. 


TERMS  IN  HUMAN  ANATOMY. 


Ganglion  Gasseri,  composed  of. 


Ganglion  geniculare, 


MORPHOLOGICAL  EQUIVALENTS. 

Ganglion     ophthalmicum     superfi- 

ciale  VII. 

Ganglion  buccale  VII. 
Ganglion  mandibulare  VII. 
Ganglion  semilunare  V. 
Ganglion  ophthalmicum  prof  undum. 

C  Sensory  ganglion  of  VII,  excepting 
J      the  parts  belonging  to  the  lateral 
|^     line  system. 


Ganglion  petrosum J  Together    form    the    sensory    gan- 

Ganglion  jugulare  IX 


glion  of  IX. 

Compound  sensory  ganglion  of  X, 
formed  by  the  fusion  of  the 
ganglia  of  all  of  the  original 
sensory  elements  with  the  excep- 
tion of  the  ganglion  laterale  of 
the  lateral  line  system. 

N.  petrosus  superficial  major Ramus  palatinus  VII. 


Ganglion  jugulare  X 


N.  palatinus  major  V. 


Continuation  of  the  ramus  pala- 
tinus VII  beyond  the  spheno- 
palatine  ganglion,  plus  some 
fibers  from  the  Trigeminus. 

Chorda  tympani N.  mandibularis  internus  VII. 

N.  tympanicus f  T°gether    f orm    Jacobson's    nerve 

N.  petrosus  superficial  minor'.  \        which  is'.  morphologically,  ramus 

(^      commnnicans  IX. 

N.  petrosus   profundus    major.   (  *oth  included  in  the  branch  of  the 
N.  petrosus  prof undus  minor..    ]       abovet°     ^     spheno-palatme 

ganglion. 


THE    NERVOUS    SYSTEM  457 

The  lingual  branch  of  the  Glosso-pharyngeus,  which  appears 
first  in  the  Dipnoi,  becomes  in  the  Amniota,  and  especially  in 
the  mammals,  a  large  and  important  nerve,  and  specializes  as 
the  nerve  of  taste  (gustatory).  The  remaining  branch  is 
motor  and  is  distributed  to  those  muscles  which  are  derived 
from  those  of  the  first  branchial  arch.  In  the  same  way  motor 
branches  of  the  Vagus  supply  the  muscles  of  the  larynx  and 
trachea,  and  the  walls  of  the  pharynx. 

The  ramus  lateralis  disappears  with  the  advent  of  terrestrial 
life,  but,  on  the  other  hand,  the  increase  in  size  and  complexity 
of  heart,  lungs,  oesophagus,  and  stomach  so  increase  the  im- 
portance of  ramus  intestinalis  that  it  alone  comes  to  be  con- 
sidered the  main  nerve  (hence  the  name  "  Pneumo-gastric  "), 
of  which  the  other  elements  are  considered  branches. 

The  circumstance  which  leads  in  Sauropsida  to  the  partial, 
and  in  mammals  to  the  complete,  separation  of  the  Accessorius 
is  clearly  found  in  the  greater  development  and  higher  degree 
of  independence  of  the  parts  to  which  it  is  supplied,  the  trape- 
zius  and  sterno-cleido-mastoid  muscles,  a  development  due 
in  its  turn  to  the  increased  importance  of  the  neck.  This 
nerve  is  still  a  part  of  the  Vagus  in  the  human  embryo  and 
shows  the  steps  of  its  gradual  emancipation  during  develop- 
ment (Fig.  125). 

V.  HYPOGLOSSUS. 

The  Hypoglossal  nerve,  which  appears  in  the  higher  verte- 
brates as  the  last  of  the  cranial  nerves,  is  plainly  one  or  more 
adopted  spinal  nerves,  found  still  in  their  original  office  in 
fishes  and  amphibians.  It  is  mainly  a  motor  nerve  and  arises 
from  several  roots  which  belong  in  the  ventral  series;  this 
is  rendered  more  certain  by  the  appearance,  usually  transi- 
tory and  embryonal,  of  corresponding  dorsal  roots,  equipped 
with  ganglia,  which  thus  complete  the  elements  necessary  for 
genuine  spinal  nerves.  In  Plate  VII  these  latter  are  indicated 
by  dotted  lines;  and  the  two  spinal  nerves  which  are  shown 
in  Plate  VI  may  be  considered  to  represent  the  potential 
hypoglossal  still  in  an  indifferent  condition.  Some  of  the 
spino-occipital  elements  may  also  enter  into  the  formation  of 


458 


HISTORY    OF    THE    HUMAN    BODY 


the  Hypoglossus,  but  this  has  not  yet  been  definitely  shown. 
The  Hypoglossus  enters  into  connection  with  one  or  two  of 
the  first  cervical  nerves,  forming  the  ansa  hypoglossi,  a  union 


gang  crest 


Opthal  div  — 
Sup  max  & 
N  masticatcrius 
Tnf  max.d/V 


FIG.  125,  a.  Reconstruction  of  peripheral  nerves  in  human  embryo. 
[After  STREETER.]  Four  weeks  human  embryo,  6.9mm  long. 

N.  tymp,  tympanic  nerve;  N.  laryg.  sup,  superior  laryngeal  nerve;  Gang,  petros, 
ganglion  petrosum;  Gang  nodos,  ganglion  nodosum;  Fronep,  Froriep's  ganglion. 
The  cranial  nerves  are  designated  by  roman  numerals,  the  spinal  by  arabic.  The 
other  designations  are  evident. 

from  which  proceed  motor  nerves  to  certain  of  the  hyoid  and 
extrinsic  laryngeal  muscles. 

It  is  now  time,  after  this  review  of  the  cranial  nerves  and 
their  morphological  history,  to  take  up  the  question  of  the 
original  segmentation  of  the  vertebrate  head,  and  consider  what 
light  the  nerves  throw  upon  this  obscure  and  much-disputed 
subject. 


THE    NERVOUS    SYSTEM 


459 


V<pgu,s  root  gong. 

Accessory  root  gang. 


/X  root  gang 


Gang,  petros 
N  tymp 


Br  to 
carotid  plexus 


Froriep 


XI. 


Sympathetic. 


FIG.    125,  b.     Reconstruction    of    peripheral    nerves    in    human    embryo. 
[After  STREETER.]     Six  weeks  human  embryo,  I7.smm  long. 
For  abbreviations  see  Fig.  125,  a. 


46o  HISTORY   OF    THE    HUMAN    BODY 

The  question  rests  upon  the  assumption  that  the  precursors 
of  the  vertebrates  were,  like  Ampkioxus,  headless  but  com- 
pletely segmented,  and  that  the  head  ivas  first  formed  by  the 
union  of  a  certain  definite,  though  probably  rather  small,  num- 
ber of  somites,  each  with  its  similar  set  of  organs,  into  a  single 
body  unit  or  complex.  A  similar  process  has  been  postulated 
in  the  case  of  the  head  of  insects,  and  the  two  cases  have  much 
that  is  analogous,  especially  the  gradual  addition  of  primarily 
trunk  somites  in  proceeding  from  lower  to  higher  forms.  In 
a  myriapod,  for  example,  or  still  better,  in  an  annelid  like  the 
earth-worm,  the  somites  are  practically  alike,  each  containing 
one  pair  of  nerve  ganglia,  one  pair  of  segmental  organs  (in  the 
latter  case),  one  set  of  metameric  muscles,  and  one  pair  of  ex- 
ternal appendages,  and  it  is  by  comparing  the  number  of  gan- 
glia, of  appendages,  or  other  metameric  parts,  that  morpholo- 
gists  attempt  to  resolve  the  head  complex  into  its  primary 
somites.  In  much  the  same  way  the  body  somites  of  Amphi- 
oxus,  or,  to  a  lesser  degree,  of  a  fish,  are  also  similar,  and  the 
pairs  of  nerves,  the  gills,  the  nephridia,  and  especially  the  myo- 
tomes,  are  metameric,  at  least  in  the  embryo.  By  thus  ascer- 
taining the  original  number  of  each  of  these  metameric  elements 
that  exist  in  the  head,  as  shown  in  the  embryological  record, 
morphologists  have  sought  here  also  to  reconstruct  the  early 
conditions  and  translate  the  head  of  modern  vertebrates  into  a 
definite  series  of  somites,  each  with  its  metameric  parts.  Thus 
far  the  views  of  investigators  are  widely  apart,  and  the  sug- 
gested number  of  primary  somites  varies  from  three  or  four  to 
eighteen,  or  even  more ;  the  most  usual  results  agree,  however, 
in  placing  the  number  between  the  limits  of  nine  and  eleven. 

The  posterior  portion  of  the  head  in  fishes,  and  especially 
in  certain  primitive  selachians,  shows  a  definite  metamerism, 
marked  in  the  cartilaginous  gill-arches,  the  arterial  arches, 
and  the  cranial  nerves  (Vagus  group),  but  anterior  to  the 
otic  region  this  becomes  effaced,  and  it  is  extremely  difficult 
to  see  here  any  suggestions  of  segmentation,  even  in  the 
embryo.  Some,  have,  indeed,  asserted  that  the  prachordal  and 
parachordal  portions  of  the  head,  divided  at  about  the  hypo- 


THE   NERVOUS    SYSTEM  461 

physis,  are  distinctly  different  in  this  respect,  and  that  while 
the  latter  forms  the  original  anterior  end  of  the  primary  seg- 
mented ancestor  and  may  thus  be  expected  to  show  traces  of 
metamerism,  the  former  or  prsechordal  portion  represents  a 
later  addition,  gained  somewhere  between  Amphioxus  and 
cyclostomes,  and  is  thus  primarily  unsegmented.  It  is  more 
probable,  however,  that  this  prsechordal  portion  is  primarily 
metameric  as  well  as  the  others,  and  that  the  indications  of 
this  have  become  more  completely  effaced,  first,  because  it 
began  to  be  modified  much  earlier  than  the  other  part,  and 
secondly,  that,  because  of  its  position,  it  became  naturally  the 
seat  of  important  organs  of  special  sense  and  became  more 
modified  through  their  influence. 

In  the  consideration  of  this  problem,  the  cranial  nerves  offer 
an  especially  hopeful  material,  as  the  various  sensory  and  mo- 
tor elements,  sensory  ganglia  and  other  parts>  suggest  that  they 
have  differentiated  from  an  original  series  of  typical  spinal 
nerves.  Thus,  as  primary  sensory  roots,  each  with  a  ganglion, 
we  may  suggest  the  Trigeminus,  Facialis,  Glosso-pharyngeus, 
and  Vagus,  the  latter  a  compound  nerve,  capable  of  resolution 
into  four,  or  perhaps,  five  elements.  If  to  these  the  ramus  oph- 
thalmicus  profundus  be  added  with  its  ganglion  as  an  origi- 
nally separate  element,  we  have  the  sensory  roots  of  eight  origi- 
nal pairs.  The  three  nerves  of  the  eye  muscles  are,  both  in 
origin  and  function,  motor  roots,  and  in  some  cases,  as  in  the 
relation  of  Abducens  to  Facialis,  they  seem  to  belong  with  cer- 
tain definite  sensory  elements.  The  tracing  out  of  prae-  and 
post-trematic  branches  which  include  a  gill-slit  as  above  men- 
tioned assists  in  locating  seven  elements,  if  the  mouth  opening 
and  spiraculum  be  included. 

The  above  relations  are  summarized  in  the  following  dia- 
gram, which,  although  not  claimed  as  the  ultimate  solution  of 
the  problem  is,  at  least,  suggestive  (Fig.  126,  A).  The  head 
is  here  represented  as  being  composed  of  nine  somites  on  the 
basis  of  nine  pairs  of  "  head  cavities  "  (muscle  somites  or  myo- 
tomes)  found  in  dog-fish  embryos.  These  are  represented  by 
the  heavy  black  rings  numbered  from  I-IX.  For  the  first 


462 


HISTORY   OF   THE    HUMAN    BODY 


somite  the  Ophthalmicus  profundus  represents  the  sensory, 
and  the  Motor  oculi  the  motor  root.  To  the  second  the  remain- 
ing portion  of  the  Trigeminus  and  the  Trochlearis  are  similarly 
related,  the  former  with  prae-  and  post-trematic  branches  about 
the  mouth.  The  third  is  supplied  by  the  Facialis  for  a  sensory 


rx 


Olf.       CiL      v     vn      vra      ix 


'  B 


G«         G*        Gj 


FIG.  126.  Two  suggestions  for  the  solution  of  the  problem  of  verte- 
brate cephalogenesis. 

(A)  According  to  VAN  WIJHE.     (B)   According  to  BEARD. 

The  roman  numerals  enclosed  in  the  ovals  in  A  designate  the  head  somites, 
all  other  roman  numerals  refer  to  the  cranial  nerves,  m,  mouth;  n,  nasal  opening; 
sp,  spiracular  cleft;  Gv  Gv  etc.,  gill-slits;  Olf,  olfactory  nerve;  Cil,  ciliary  nerve. 

and  by  the  Abducens  for  a  motor  root,  but  for  the  fourth  we 
have  the  Acusticus  alone,  with  the  motor  root  wanting.  The 
remaining  five  possess  the  five  elements  shown  in  the  Vagus 
group  in  fishes,  while  two  Hypoglossus  roots  supply  motor  ele- 
ments for  the  last  three  somites.  Fig.  126,  B,  shows  a  sug- 
gestion made  by  another  investigator,  and  based  mainly  upon 


THE    NERVOUS    SYSTEM  463 

the  relation  of  prse-  and  post-trematic  branches  to  their  cor- 
responding gill-slits. 

Later  researches  have  modified  these  diagrams  somewhat, 
as,  for  example,  the  discovery  of  transitory  sensory  roots  for 
the  hypoglossal  elements,  and  indications  of  other  nerves  an- 
terior to  the  Ophthalmicus  profundus;  the  spino-occipital 
nerves,  although  of  unknown  value,  seem  indicative  of  still 
other  somites  in  the  occipital  region  and  need  to  be  thoroughly 
explained  before  the  problem  of  the  segmentation  of  the  head 
can  receive  its  final  solution. 

There  remains  to  be  mentioned  an  auxiliary  system  of  gan- 
glia and  nerve  fibers,  not  directly  under  the  control  of  the  will, 
but  often  of  great  importance  in  regulating  the  physiological 
activities  of  certain  of  the  internal  organs.  This  is  the  sym- 
pathetic system,  often  erroneously  treated  as  originally  a  dis- 
tinct nervous  system  coordinate  with  the  cerebro-spinal,  thus 
far  the  subject  of  this  chapter.  As  a  matter  of  fact,  however, 
the  sympathetic  system  is  an  integral  portion  of  the  latter,  and 
its  differentiation  from  this  may  be  followed  both  in  the  race 
history  as  well  as  during  individual  development.  It  attains 
its  greatest  degree  of  individuality  only  among  the  higher 
forms.  It  consists  primarily  of  a  series  of  ganglia,  segmented 
off  from  the  sensory  ganglia  of  the  spinal  nerves,  although  re- 
taining connection  with  their  places  of  origin  through  commu- 
nicating branches.  The  ganglia  of  the  two  sides,  which  come 
to  lie  ventral  to  the  spinal  nerves  on  either  side  of  the  ver- 
tebral column,  may  become  secondarily  connected  with  one  an- 
other by  longitudinal  connectives,  thus  forming  two  lateral 
trunks. 

This  appearance  of  metameric  ganglia  connected  in  two  lon- 
gitudinal series,  and  especially  their  ventral  position,  has  led 
the  sympathetic  system  to  be  compared  to  the  ventral  chain 
of  ganglia  found  in  articulates  (e.g.,  insects,  crustaceans),  a 
suggestion  of  homology  that  is  sufficiently  disproved  by*  the 
mode  of  origin  and  the  fact  that  the  similarity  is  most  perfect 
in  the  higher  forms.  Indications  suggest  that  the  separation 
of  sympathetic  ganglia  began  historically  in  the  head,  since  in 


464  HISTORY    OF    THE    HUMAN    BODY 

fishes  the  system  is  better  developed  in  this  region  than  in  the 
trunk.  In  the  embryo  also  the  cephalic  portion  develops  be- 
fore the  rest. 

The  system  appears  well  developed  in  Amphibia,  with  its 
two  lateral  trunks.  In  Sauropsida  a  pair  of  subsidiary  trunks 
in  the  neck  region  accompanies  the  vertebral  arteries,  the  only 
instance  of  a  distinct  dorsal  position  for  any  part  of  this  system. 

From  the  ganglia  as  centers  numerous  nerve  fibers  proceed, 
supplying  many  of  the  internal  organs,  especially  the  alimen- 
tary canal  and  the  arteries,  the  favorite  mode  of  distribution 
being  an  intricate  plexus,  which  spreads  over  the  broader  sur- 
faces and  enwraps  the  smaller  parts. 

Owing  to  the  origin  of  the  sympathetic  system  from  the 
strictly  metameric  sensory  ganglia  of  the  cerebro-spinal  nerves, 
this  system  also  shows  at  first  a  metameric  character;  this  ap- 
pearance becomes  modified,  however,  in  regions  of  the  greatest 
differentiation,  as  in  the  head  and  neck  and  the  pelvic  region. 
The  four  cephalic  ganglia,  ciliary,  spkeno-palatine,  otic,  and 
submaxillary,  which  in  mammals  assist  in  forming  connec- 
tions between  certain  of  the  cranial  nerves,  and  which  have 
been  treated  with  these  latter  parts,  belong  to  the  sympathetic 
system. 


CHAPTER   XI 
THE   SENSE-ORGANS 

"  Die  wunderbare  und  wirklich  iiberraschende 
Ahnlichkeit  in  der  inneren  Organisation,  in  den 
anatomischen  Structurverhaltnissen,  und  die  noch- 
merkwiirdigere  Ubereinstimmung  in  der  embryonalen 
Entwickelung  bei  alien  Thieren,  welche  zu  einem  und 
demselben  Typus,  z  B.,  zu  dem  Zweige  der  Wirbel- 
thiere,  gehoren,  erklart  sich  in  der  einfachsten  Weise 
durch  die  Annahme  einer  gemeinsamen  Abstammung 
derselben  von  einer  einzigen  Stammform.  Entschliesst 
man  sich  nicht  zu  dieser  Annahme  so  bleibt  jene  durch- 
griefende  Ubereinstimmung  der  verschiedensten  Wir- 
belthiere  im  inneren  Bau  und  in  der  Entwickelungs- 
weise  vollkommen  unerklarlich." 

ERNST  HAECKEL,  Schopfungsgeschichte,  Kap.  III. 

IT  will  be  remembered  that  the  nervous  system  is  primarily 
external,  developed  in  response  to  stimuli  from  without,  and 
that,  as  this  system  becomes  more  specialized,  and  hence  of 
greater  importance  to  the  organism,  it  withdraws  in  great  part 
into  the  interior,  leaving  upon  the  surface  a  set  of  sense-organs, 
capable  of  receiving  the  impressions  and  transmitting  them  to 
the  central  organ.  Taking  into  consideration  the  intimate  con- 
nection between  these  two  portions,  external  and  internal, 
it  might  be  supposed  that  the  enormous  development  in  size 
and  complexity  shown  by  the  brain  would  be  the  result  of  a 
corresponding  degree  of  differentiation  of  the  external  parts, 
yet  such  is  by  no  means  the  case.  The  sense-organs  are  early 
brought  to  a  high  state  of  efficiency  and  develop  but  little 
during  the  entire  vertebrate  history.  The  eye  of  the  fish  is 
almost  as  good  an  optical  instrument  as  is  that  of  the  mam- 
mal, and,  save  for  a  few  external  parts,  is  as  complex;  the 
sense  of  hearing,  although  not  as  early  in  development  as  the 
eye,  is  yet  very  acute  in  reptiles,  and,  perhaps,  in  amphibians, 
and  gains  in  birds  and  mammals  very  little  except,  perhaps, 

465 


466  HISTORY   OF   THE    HUMAN    BODY 

the  recognition  of  musical  tones;  indeed,  the  early  aquatic 
forms  possess  in  the  lateral  line  organs  an  entire  system,  no 
trace  of  which  seems  to  have  survived  the  transition  to  land, 
and  yet,  with  no  especial  progress  on  the  part  of  the  sense- 
organs,  the  central  nervous  system,  and  especially  the  brain, 
the  receiving  organ  of  the  special  senses,  has  increased  from 
a  simple  condition  to  one  showing  a  marvelous  degree  of 
complexity.  The  cause  of  this  extreme  development  must  be 
laid,  then,  not  to  the*  sense-organs,  but  to  the  direct  and  cumu- 
lative influence  of  the  impressions  received.  The  motor  cen- 
ters, which  have  contributed  not  a  little  to  the  complexity  of 
the  central  nervous  system,  have  also  developed  in  response 
to  the  external  environment,  though  rather  more  indirectly, 
perhaps,  through  the  necessity  of  controlling  the  more  spe- 
cialized limbs  and  other  parts,  which,  in  their  turn,  were 
directly  influenced  by  external  conditions. 

The  morphological  history  of  the  sense-organs  does  not, 
therefore,  show  the  extensive  progress  exhibited  in  the  case 
of  most  of  the  other  systems;  but  as  certain  definite  changes 
were  necessitated  by  the  transition  from  water  to  land,  this 
history  is  divided  into  two  great  stages,  ( i )  that  of  the  aquatic, 
and  (2)  that  of  the  terrestrial  life. 

Throughout  the  animal  kingdom,  the  elementary  type  of 
sense-organ  is  a  single  epithelial  cell,  connected  by  a  nerve 
with  some  sensory  center.  From  this  as  a  starting  point 
higher  efficiency  is  gained  in  three  ways:  (i)  by  the  asso- 
ciation of  a  number  of  these  elementary  units  to  form  a  larger 
sensory  area,  (2)  by  the  specialization  of  the  cell  itself,  and 
(3)  by  the  development  of  accessory  parts. 

The  area  over  which  a  given  form  of  sensory  cell  may  occur 
may  be  a  general  surface  of  indefinite  limits,  or  it  may  be 
restricted  and  form  a  definite  sense-organ.  Most  generally 
the  epithelial  cells  composing  such  an  area  are  not  homogene- 
ous, but  are  differentiated  among  themselves  into  two  sorts, 
sensory  cells  and  supporting  cells;  the  first  are  the  receptive 
units  of  the  nervous  system ;  the  latter  are  non-sensitive,  and 
are  grouped  about  the  sensory  cells  in  such  a  way  as  to  form 


THE    SENSE-ORGANS  467 

a  support  and  protection  for  the  essential  elements  of  the  sense- 
organ,  which  may  thus  attain  a  high  degree  of  sensitiveness. 

Regarding  the  differentiation  of  the  sensory  cells  themselves, 
they  may  present  at  their  free  end  a  ciliated  or  simple  sur- 
face, or  may  bear  one  or  more  flagella.  In  the  most  special- 
ized types  the  flagella  themselves  may  be  modified  for  the  better 
reception  of  certain  definite  forms  of  impression,  as  in  the 
case  of  the  rods  and  cones  of  the  retina,  or  the  acoustic  hairs 
of  the  inner  ear;  these  types,  however,  owing  to  the  extreme 
delicacy  of  the  projecting  parts,  can  exist  only  upon  an  external 
surface  bathed  by  water,  as  in  many  aquatic  invertebrates,  or 
upon  a  surface  that  faces  some  internal  cavity  furnished  with 
an  artificial  fluid  or  semi-fluid. 

Accessory  organs  for  the  reception  and  intensification  of  the 
external  stimuli  or  for  the  protection  and  care  of  the  essential 
parts  are  the  rule  in  the  case  of  the  more  specialized  and  com- 
plex organs,  but  are  not  employed  to  assist  in  the  reception  of 
general  tactile  impressions  save  in  certain  invertebrates  with 
a  thick  and  hard  exo-skeleton  which  would  naturally  prevent 
such  impressions  from  reaching  the  interior.  In  this  latter 
case  the  sensations  are  transmitted  by  sensory  hairs  which  are 
protruded  through  pores  in  the  external  armor,  and  communi- 
cate with  underlying  sense-organs.  In  those  vertebrates  in 
which  the  integument  is  covered  by  non-sensitive  parts,  such 
as  scales,  feathers,  or  hairs,  often  necessary  to  protect  the  ani- 
mal from  serious  injury,  the  sensory  organs  are  developed 
between  them,  or  may  be  situated  about  their  bases,  when  they 
are  stimulated  indirectly  through  the  movements  of  the  insen- 
sitive outer  parts,  much  as  in  the  previous  case. 

Concerning  the  actual  sensations  produced  by  the  different 
kinds  of  sense-organs  found  among  vertebrates  we  know  very 
little,  and  inferences  must  be  made  with  extreme  caution.  Al- 
though something  can  be  deduced  from  the  mechanical  struc- 
ture of  a  terminal  organ,  especially  from  that  of  its  accessory 
parts,  and  although  from  the  physiological  side  something  can 
be  learned  from  the  behavior  and  responses  of  an  animal  under 
observation,  the  only  certainty  concerns  our  own  sense-organs 


468  HISTORY    OF    THE    HUMAN    BODY 

and  those  formed  like  them.  Thus,  we  are  positive  concerning 
the  sense  furnished  by  the  eye,  since  the  variations  from  the 
human  structure  are  very  slight  in  any  case,  even  in  fishes; 
the  ear,  however,  is  somewhat  more  variable,  and  presents  sev- 
eral problems,  since  the  lower  types  of  ear  lack  certain  parts, 
like  the  cochlea,  which  in  Man  are  essential  to  the  complete- 
ness of  the  sense  of  hearing  as  we  understand  it.  In  this 
case  we  have  two  alternatives,  either  that  in  the  different  forms 
the  same  function  is  subserved  by  different  parts,  as  is  possible 
in  the  case  of  the  brain  (cf.  the  pallium  of  teleosts  and  the 
cerebral  hemispheres  of  mammals),  or  else  that  there  are  ele- 
ments in  the  human  sense  of  hearing  not  perceived  by  ears  be- 
longing to  other  types. 

Aside  from  the  above,  the  sense  of  smell  seems  to  be  a  com- 
mon possession,  and  in  terrestrial  forms  the  sense  of  taste  also 
seems  general  if  we  are  to  judge  from  the  similarity  in  the 
location  and  structure  of  certain  specialized  sense-organs  and 
the  identity  of  their  nerve  supply.  After  these  are  excepted, 
however,  there  remains  a  large  number  of  types  of  terminal 
sense-organs,  more  or  less  localized  in  different  areas,  the  spe- 
cial functions  of  which  are  practically  unknown,  but  are  in- 
cluded within  the  comprehensive  terms  of  touch  or  feeling. 
That  many  distinct  impressions  are  involved  in  this  is  shown 
by  this  very  dissimilarity  in  the  structure  of  the  terminal  or- 
gans, the  complexity  of  which,  in  certain  cases,  suggests  the 
possibility  of  definite  senses,  at  least  as  distinct  as  those  of  smell 
or  taste ;  but  as  few  of  these  types  occur  in  Man,  and  as  even 
here  the  elementary  sensations  have  not  been  wholly  coordi- 
nated with  the  various  forms  of  nerve  terminations,  but  little 
can  yet  be  stated  on  the  subject,  and  the  psychology  of  the 
tactile  sensations  of  the  lower  vertebrates  remains  an  unex- 
plored field. 

Probably  the  lowest  form  of  vertebrate  sense-organ,  and  one 
that  is  universally  distinguished  among  them,  is  that  of  simple 
sensation,  the  contact  sense,  which  resides  in  the  epidermis 
and  is  thus  generally  met  with  over  the  entire  surface.  The 
vertebrate  epidermis,  which,  in  contrast  to  that  of  invertebrates, 
is  many  cells  thick,  is  supplied  everywhere  by  sensory  nerves 


THE    SENSE-ORGANS  469 

which  branch  repeatedly  and  with  their  ultimate  fibers  form 
a  delicate  net-work  which  permeates  the  entire  layer,  stopping 
only  at  the  most  external  cells.  This  type  is  characteristic  of 
all  vertebrate  integument,  including  both  cyclostomes  and 
mammals,  and  furnishes  them  all  with  a  surface  capable  of  re- 
sponding to  general  tactile  impressions. 

Aside  from  this  general  tactile  sense  vertebrates  possess  a 
large  number  of  more  or  less  specialized  sensory  endings, 
usually  classed  also  as  tactile.  These  are  local  in  distribution, 
often  confined  to  a  single  group  of  animals,  and  usually  oc- 
cur upon  prominent  portions  of  the  body  or  occasionally,  as 
in  the  case  of  certain  fishes,  upon  special  papillae  or  long  fila- 
ments. The  most  extensive  of  these,  and  the  only  one  to 
develop  into  a  definitely  organized  system,  is  that  of  the  lateral 
line  organs,  referred  to  above  in  connection  with  the  cranial 
nerves  and  possessed  by  the  primarily  aquatic  vertebrates 
(fishes  and  amphibians).  Although  this  system,  as  such,  to- 
gether with  its  nerves,  disappears  utterly  with  the- assumption 
of  a  terrestrial  life,  it  is  yet  of  importance  in  this  connection 
because  of  the  possible  derivatives  from  it  in  higher  forms, 
among  which  have  been  mentioned,  with  more  or  less  basis 
for  the  claim,  the  taste-buds,  the  inner  ear,  and  the  mammalian 
hair. 

In  its  simplest  form  a  lateral  line  organ  consists  of  a  small 
group  of  sensory  cells,  slightly  convex  in  form  and  protected 
by  a  wall  of  non-sensitive  supporting  cells.  This  organ  gains 
its  simplest  form  of  protection  by  sinking  slightly  beneath 
the  surface,  its  supporting  cells  remaining  at  the  general  level, 
or  even  projecting  a  little  above  it.  By  continuing  this  process 
the  sense-organ  comes  to  lie  at  the  bottem  of  a  flask-shaped 
cavity,  communicating  with  the  surface  by  a  narrow  neck. 

From  this  point  on,  greater  complexity  may  be  gained  by 
development  in  one  of  two  directions,  the  flask-shaped  cavities 
may  either  become  associated  in  rows  and  break  down  their 
adjacent  walls,  forming  the  slime  canals,  or  else  each  separate 
flask  may  become  elongated,  bearing  the  sense-organ  at  its 
very  bottom,  as  in  the  canals  of  Lorenzini.  In  the  first  of 
these,  the  coalescence  may  result  in  the  formation  of  either  a 


470 


HISTORY   OF   THE    HUMAN    BODY 


THE    SENSE-ORGANS  471 

deep  trough  or,  more  usually,  an  enclosed  tube,  running  just 
below  the  surface ;  and  in  this  latter  case,  each  organ  may  open 
by  its  own  pore,  placed  directly  above  it,  or  an  entire  tube 
may  open  by  a  single  common  pore  at  one  end.  These  canals 
are  usually  filled  with  a  clear  mucous  or  gelatinous  material, 
secreted  by  the  walls,  and  destined  to  protect  the  sensory  cells. 
In  the  case  of  the  second  form  of  development,  there  will  be 
produced  local  groups  of  associated,  though  distinct,  canals, 
each  beginning  superficially  at  an  external  pore  and  running 
obliquely  beneath  the  surface  to  terminate  proximally  in  a  bulb 
or  ampulla,  in  which  the  sense-organ  is  located.  These  organs 
occur  in  localized  masses  in  the  heads  of  selachians,  associated 
topographically  with  the  mucous  canal  type,  the  ampullae  with 
their  nerves  clustered  in  such  a  way  as  to  resemble  bunches  of 
grapes  (Fig.  127). 

That  a  system  so  highly  developed  and  so  extensive  in  its 
distribution  as  that  of  the  lateral  line  organs  should  have 
wholly  disappeared  in  terrestrial  vertebrates,  together  with  its 
nerves,  is  a  phenomenon  of  so  unusual  a  nature  that  numerous 
attempts  have  been  made  by  morphologists  to  find  its  direct 
continuation  among  the  parts  of  higher  vertebrates.  Thus,  one 
well-known  theory  associates  these  organs  with  the  taste-buds, 
a  view  arising  naturally  from  the  extreme  similarity  between 
the  two  structures.  There  seem,  ho\vever,  to  be  no  definite 
data  to  form  the  logical  steps  between  the  two,  and  the  fact 
that  the  nerve  supply  to  the  taste-buds  comes  from  the  Glosso- 
pharyngeus  and  not  from  any  part  of  the  extensive  sensory 
system  associated  with  the  lateral  line  organs  speaks  strongly 
against  this  homology.  As  an  added  evidence  in  the  same  direc- 
tion there  are  found  in  the  nasal  mucous  membrane  of  many 
fishes  and  of  certain  of  the  lowest  urodeles  (Siren)  groups  of 
cells  forming  (f  smell-buds/'  extremely  similar  to  taste-buds, and 
yet  by  no  possibility  connected  with  the  lateral  line  system. 

A  second  possible  survival  of  the  lateral  line  organs  is 
seen  by  some  in  the  hair  of  mammals.  This  theory  is  based 
upon  a  certain  similarity  in  the  early  stages  of  development 
of  the  two  structures,  that  is,  the  initial  procedure  in  both 


472  HISTORY   OF   THE    HUMAN    BODY 

cases  concerns  the  epidermis  alone  and  consists  of  a  concentric 
arrangement  of  a  small  group  of  cells.  If  this  homology  be 
a  true  one  we  must  also  consider  the  amphibians  as  the  direct 
ancestors  of  mammals,  since  the  lateral  line  organs  do  not 
occur  in  reptiles.  In  this  comparison  the  original  sense-organ 
is,  of  course,  the  equivalent  of  the  convex  hair  papilla,  which 
lies  at  the  root,  covering  the  corium  papilla,  and  from  which 
proliferate  the  cornified  cells  of  the  hair  shaft.  Something 
analogous  to  this  exists  in  the  so-called  "  pearl-organs,"  horny 
bodies  which  develop  from  certain  of  the  lateral  line  organs 
in  some  fishes,  showing  that  there  is  present  in  these  organs 
a  tendency  to  produce  cornified  structures. 

Again,  the  sensitiveness  of  the  hair  root,  and  its  abundant 
nerve  supply,  especially  in  cases  like  that  of  the  vibrisscz  (whis- 
kers) of  the  upper  lip  in  many  mammals,  speaks  in  favor  of 
such  a  derivation.  The  great  multiplicity  of  the  hairs,  con- 
sidering that  each  represents  an  original  sense-organ,  and  also 
their  almost  universal  distribution,  is  paralleled  by  the  adap- 
tive multiplication  of  other  parts,  such  as  the  mammae  in  some 
forms  or  the  vertebrae  in  elongated  animals.  On  the  other 
hand,  the  test  of  nerve  supply  fails  to  even  suggest  this  hypoth- 
esis, since  in  the  earliest  terrestrial  vertebrates  the  extensive 
system  of  superficial  nerves  associated  with  the  lateral  line 
organs  becomes  entirely  lost  (unless  the  Acusticus  may  be 
looked  upon  as  derived  from  it)  ;  furthermore,  the  close  asso- 
ciation between  hairs,  scales,  and  integumental  glands  turns 
the  argument  in  a  totally  different  direction.  (Cf.  Chap.  IV.) 

The  third  possible  derivative  of  the  lateral  line  system,  the 
inner  ear,  does  not  come  into  the  same  category  as  the  taste- 
buds  and  the  hair,  since  if  it  came  from  this  system  at  all,  it 
must  have  separated  from  it  very  early,  and  thus  could  not  in 
any  case  be  considered  a  survival  of  the  system  as  it  exists  in 
fishes  and  amphibians.  This  theory  receives  its  strongest  sup- 
port from  the  developmental  origin  of  the  Eighth  nerve,  which 
has  been  clearly  proven  to  segment  off  from  that  part  of  the 
Seventh  which  supplies  the  lateral  line  organs,  certainly  a 
strong  argument,  since,  if  the  Eighth  nerve  were  once  an  ele- 


THE    SEXSE-ORGANS  473 

ment  of  this  system,  the  part  to  which  it  is  distributed  must 
have  been  so  also.  A  suggestive  comparison  has  also  been 
made  between  the  semicircular  canals  of  the  inner  ear,  each 
with  its  own  ampulla,  and  the  canals  of  Lorenzini,  the  resem- 
blance between  which  is  apparent.  The  actual  value  of  this 
comparison  is  somewhat  questionable,  and  the  theory  itself,  al- 
though far  better  supported  by  the  facts  than  are  either  of 
the  others,  is  seriously  opposed  by  the  actual  developmental 
history  of  the  labyrinth ;  which  arises  as  a  vesicle  invaginated 
from  the  exterior  long  previous  to  and  not  associated  with  the 
lateral  line  organs.  The  otic  vesicle  seems  rather  to  form  one ' 
of  a  series  of  very  early  organs,  to  which  belong  also  the  lens 
of  the  eye  and  possibly  the  nasal  sacs,  as  well  as  a  few  transi- 
tory structures  associated  with  other  cranial  nerves  and  usually 
interpreted  as  lost  sense-organs  of  unknown  function.  In  our 
present  state  of  knowledge  it  seems  a  surer  course  to  believe 
that  the  entire  lateral  line  system  of  the  Ichthyopsida,  the 
function  of  which  is  in  some  way  associated  with  an  aquatic 
habitat,  disappears  completely  where  the  assumption  of  a  ter- 
restrial life  renders  it  no  longer  necessary. 

In  connection  with  the  description  of  the  general  tactile 
sense,  that  of  the  so-called  "  free  nerve  endings,"  certain  more 
specialized  forms  of  nerve  termini  were  referred  to,  which  in 
our  present  lack  of  precise  knowledge  are  classed  under  the 
general  head  of  organs  of  touch.  The  most  elementary  of  these 
are  the  tactile  cells  (Fig.  128,  b),  which  are  scarcely  more  than 
isolated  units  of  the  general  type,  somewhat  more  specialized 
and  thus  rendered  conspicuous.  They  are  first  seen  in  tailless 
amphibians,  where  they  are  associated  in  groups,  forming  small 
areas  known  as  tactile  spots.  In  other  cases  the  ending  has  a 
tendency  to  form  a  bulb  or  sphere,  composed  of  many  cells, 
and  often  of  appreciable  size;  these  are  termed  collectively 
tactile  corpuscles,  each  different  type  being  designated  by  the 
name  of  the  investigator  who  first  made  an  accurate  description 
of  it.  These  tactile  corpuscles  show  various  types  of  structure 
and  make  use  of  very  different  mechanical  principles.  Thus, 
in  Meissner's  corpuscles  (Fig.  128,  c)  the  terminal  cells  form 


HISTORY   OF   THE   HUMAN    BODY 

an  oval  core,  upon  which  the  nerve  fibers  are  wound  in  an 
irregular  branching  spiral.  Certain  types,  on  the  other  hand, 
seem  to  possess  no  epidermal  elements,  as  in  Krause's  cor- 
puscles (Fig.  128,  d),  which  consist  of  a  globular  snarl  of 
nerve  fibers  like  a  capillary  glomerulus,  enclosed  within  a  thin 
covering  of  connective  tissue.  In  still  another  type,  shown  by 
Grandry's  corpuscles  (Fig.  128,  e),  the  nerve  terminus  is  en- 
closed between  two  large  epidermal  cells  which  seem  to  pro- 


FIG.   128.     Various  endings  of  sensory  nerves. 

(a)    Free    nerve    ending.       (b)    Merkel's    corpuscles.       (c)    Meissner's    corpuscle, 
(d)    Krause's    corpuscle;,        (e)    Grandry's    corpuscle.        (f)    Pacini's    corpuscle. 

duce  the  stimulus  by  transmitting  any  pressure  to  which  they 
are  subjected.  This  last  principle  is  employed  in  a  more 
elaborate  manner  in  Pacini's  corpuscles,  perhaps  the  largest 
and  most  complex  of  the  series,  where  the  nerve  terminus  is 
enclosed  first  by  a  layer  of  epithelial  cells,  then  by  a  series 
of  consecutive  lamellae  of  connective  tissue,  something  like  the 
coats  of  an  onion,  and  finally  by  an  external  connective  tissue 
wrapping,  the  continuation  of  the  nerve  sheath  or  neurilemma. 


THE    SENSE-ORGANS  475 

All  of  the  above  types  of  tactile  corpuscles  occur  in  man 
and  other  mammals  with  the  exception  of  Grandry's  corpuscles, 
which  are  found  only  in  the  beaks  of  various  birds.  They  do 
not  seem  to  occur  over  the  general  hair-covered  surface,  but 
are  found,  often  in  association  with  one  another,  upon  such 
modified  hairless  surfaces  as  the  palms  and  soles,  the  tips  of 
the  digits,  the  lips,  the  nipples,  the  external  genitals,  and,  in 
many  mammals,  on  the  end  of  the  nose  or  snout.  The  Pacinian 
corpuscles  also  occur  in  such  various  internal  organs  as  the 
pancreas,  the  submandibular  gland,  and,  in  the  cat,  even  in  the 
mesentery.  This  internal  distribution  has  led  to  doubt  con- 
cerning the  function  of  these  corpuscles  as  tactile  organs,  but, 
on  the  other  hand,  their  profuse  occurrence  and  large  size  in 
such  places  as  the  balls  of  fingers  and  toes  can  be  accounted 
for  in  no  other  way. 

The  sense  of  taste,  which  in  popular  estimation  is  raised  to 
the  value  of  one  of  the  special  senses,  is,  all  things  considered, 
but  little  more  than  a  tactile  sensation,  and  the  organs  in  which 
it  is  located  are  but  little  differentiated  from  certain  of  the 
foregoing.  The  organs  of  taste  themselves,  cut  off  from  all 
association  with  the  sense  of  smell,  are  restricted  in  function 
to  the  perception  of  certain  elementary  qualities  of  liquid 
substances,  as  sweet,  sour,  bitter,  and  salt,  qualities  which  are 
mechanical  or  chemical  in  their  action  and,  as  such,  can  be  also 
perceived  and,  in  part,  distinguished,  by  the  general  tactile 
sense.  To  prove  this  last  it  is  only  necessary  to  bring  some 
acid  or  astringent  liquid  in  contact  with  a  surface  from  which 
the  external  layer  of  the  epidermis  has  been  removed. 

The  ultimate  organs  of  the  sense  of  taste,  the  taste-buds  or 
taste-beakers,  are  found,  from  the  amphibians  on,  only  within 
the  cavity  of  the  mouth,  especially  upon  the  tongue  and  palate ; 
but  in  fishes  they  are  far  more  general  in  their  distribution, 
and  have  been  found  in  some  species  (e.g.,  bull-heads)  scat- 
tered over  the  skin  of  the  external  surface.  Such  fishes  are 
thus  probably  enabled  to  taste  the  water  through  which  they 
pass.  A  taste-bud  consists  of  a  group  of  long,  spindle-shaped 
cells,  surrounded  by  a  rampart  of  supporting  cells  of  shape 


476  HISTORY   OF   THE    HUMAN    BODY 

similar  to  the  others,  but  longer,  and  thus  greatly  resemble 
the  terminal  organs  of  the  lateral  line  system.  At  their  free 
ends  the  sensory  cells  usually  possess  one  or  more  modified 
flagella,  which  project  into  a  small  space,  that  is  formed  about 
them  by  the  supporting  cells,  and  communicates  with  the  ex- 
terior through  a  small  opening.  In  mammals  the  taste-buds 
are  associated  together  in  groups  in  connection  with  several 
sorts  of  papillae,  especially  the  circumvallate,  and  the  foliate, 
the  latter  not  occurring  in  Man. 

The  sense  of  smell  is  located  in  a  pair  of  ectodermic  cavities, 
situated  anterior  to  the  eyes,  thus  forming  the  most  anterior 
of  the  sense  organs.  They  are  thus  in  the  most  favored  po- 
sition for  organs  of  sense,  and  although  the  data  are  too  in- 
sufficient for  theories,  this  fact  suggest^  that  they  were  the 
earliest  to  develop  and  that  the  primaeval  habitat  was  either  in 
mud  or  in  the  deep  sea  where  the  olfactory  sense  was  of  pri- 
mary importance.  Amphioxus  gives  no  clew  to  this,  for  here 
the  sense  of  smell  is  located  in  a  median  ciliated  pit  at  the 
anterior  end  and  pushed  a  little  to  the  left  side  by  the  develop- 
ment of  the  median  fin.  The  early  stages  of  the  cyclostomes 
furnish  much  material  for  speculation,  but  unfortunately  there 
is  no  certainty  felt  as  yet  concerning  the  meaning  of  the  details 
presented.  Here  (Fig.  129)  there  appear  at  the  anterior 
end  two  median  invaginations,  the  more  posterior  of  which  is 
the  cavity  of  the  mouth  (stomatod&um).  The  anterior  de- 
pression is  that  of  a  median  nasal  cavity,  which  would  suggest 
a  primitive  condition  and  possibly  a  kinship  with  the  ciliated 
pit  of  Amphioxus  were  it  not  for  the  fact  that  it  is  supplied  by 
two  olfactory  nerves  from  as  many  olfactory  lobes,  showing 
that  the  single  or  monorrhine  condition  has  here  been  secondar- 
ily attained  from  a  previous  paired  (amphirrhine)  one.  From 
the  posterior  wall  of  this  depression  there  develops  a  tubular 
process,  which,  in  Myxine,  connects  ultimately  with  the 
pharynx  and  thus  forms  a  direct  communication  between  nose 
and  throat,  but  in  the  other  cyclostomes  ends  blindly  and  soon 
disappears.  This  passage  is  of  interest  as  a  prophecy  of  the 
similar  connection  to  develop  later  in  air-breathing  vertebrates, 


THE    SENSE-ORGANS 


477 


and  is  of  still  greater  interest  for  its  direct  connection  with  the 
hypophysis,  which  develops  from  it.  //  in  this  may  be  seen 
the  remains  of  an  earlier  entrance  into  the  alimentary  canal, 


FIG.  129.  Median  sagittal  sections  through  the  head  of  two  stages  of 
the  Ammoccetes  embryo  of  Petromyzon.  [After  von  KUPFFER.] 

I,  II,  III,  the  three  primary  cerebral  vesicles;  d,  intestine  (mesodaeum) ; 
m,  mouth  cavity  (stomatodaeum) ;  nc,  notochord;  ep,  epiphysis;  hy,  hypophysis; 
hy',  portion  detached  from  distal  end  of  hypophysis;  rg,  olfactory  pit;  lo,  median 
olfactory  lobe;  ch,  optic  chiasma. 


478  HISTORY   OF   THE    HUMAN    BODY 

then  its  associated  mouth  (palaostoma)  must  have  been  the 
later  nasal  cavity,  and  its  unpaired  condition  in  cyclostomes,  in 
spite  of  the  contradictory  testimony  of  the  olfactory  nerves, 
would  then  be  primitive  and  not  secondary. 

In  all  other  vertebrates  the  nose  is  strictly  amphirrhine,  that 
is,  it  consists  of  two  lateral  cavities,  symmetrically  placed. 
These  cavities  begin  as  localized  thickenings  of  the  ectoderm, 
which  invaginate  and  form  nasal  sacs,  a  condition  that  per- 
sists in  fishes.  In  the  Dipnoi,  the  first  air-breathers,  these  sacs 
become  prolonged  posteriorly  and  break  through  into  the 
mouth  cavity,  forming  the  choana  [posterior  nares'],  forma- 
tions which  are  present  also  in  amphibians  and  all  higher  ver- 
tebrates. When  these  are  present,  the  lower  part  of  the  cavity 
is  used  more  or  less  exclusively  for  respiration,  and  the  olfac- 
tory sense  becomes  limited  to  the  more  dorsal  portion,  thus 
dividing  the  cavity  into  a  pars  respiratoria  and  a  pars  olfactoria. 

Within  possible  limits  the  greatest  diversity  exists  in  the 
location  of  the  nasal  cavities,  and  especially  their  openings, 
the  anterior  and  posterior  nares.  The  former  may  be  placed 
ventrally,  as  in  dog-fish,  and  may  occupy  all  intermediate 
positions  to  an  extreme  dorsal  one.  Marked  adaptations  in 
this  respect  are  seen  in  those  air-breathers  which  have  become 
secondarily  aquatic,  enabling  them  to  breath  at  the  top  of  the 
water  without  being  seen.  Thus  in  the  whales  and  porpoises 
the  nostrils  seem  to  be  moved  to  the  top  of  the  head,  the 
deception  being  due  to  very  short  frontal  and  nasal  bones  and 
to  an  excessive  anterior  prolongation  of  the  maxillaries  and 
premaxillaries.  In  some  birds,  like  the  albatrosses  and  pe- 
trels, the  nostrils  become  prolonged  into  tubes  formed  by  the 
beak,  so  that  they  open  near  the  tip  of  that  organ  instead  of 
at  its  base.  An  extreme  case  is  seen  in  the  Dipnoi,  in  adap- 
tation to  their  annual  hibernation  within  a  cocoon  of  dry 
clay;  for  in  these  animals  the  openings  of  the  anterior  nares 
lie  within  the  mouth  cavity,  and  the  mouth  is  connected  with 
the  exterior  during  hibernation  by  means  of  a  long  tube  com- 
posed of  slime  secreted  by  the  animal. 

As  regards  the  choanae,  their  original  position  is  shown  by 


THE    SENSE-ORGANS  479 

amphibians  to  be  very  far  forward,  a  position  retained,  with 
some  variation,  by  Sauropsida.  In  mammals  this  becomes 
greatly  modified  by  the  formation  of  the  hard  palate,  which 
develops  from  the  fusion  of  two  lateral  shelves,  beginning 
anteriorly.  This  shuts  off  from  the  mouth  cavity  its  own 
primary  roof,  including  the  openings  of  the  choanae,  and 
therefore  pushes  back  their  communication  with  the  mouth 
cavity  to  its  posterior  limit.  A  trace  of  the  former  communi- 
cation is  retained,  however,  in  many  mammals  in  the  form  of 
the  naso-palatine  canal  (Stenson's  canal),  which  opens  into 
the  roof  of  the  mouth  behind  the  incisor  teeth.  A  rudiment 
of  this  duct  occasionally  occurs  in  man,  lodged  in  the  incisive 
canal  of  the  maxillaries. 

For  greater  efficiency  the  olfactory  surface  may  be  increased 
in  three  ways :  ( i )  by  folding  the  nasal  mucous  membrane  in 
an  oval  or  otherwise  simple  cavity,  (2)  by  complicating  the 
walls  of  the  cavity  itself,  usually  by  means  of  ridges  or  shelves 
which  may  themselves  become  rolled  or  variously  convoluted, 
or  (3)  by  the  addition  of  accessory  cavities  within  the  ad- 
jacent bones.  The  first  of  these  devices  is  seen  in  fishes,  where 
the  folds  are  variously  disposed,  either  transverse  or  longi- 
tudinal. This  folding  of  the  mucous  surface  may  become 
extremely  complex  and  thus  furnish  an  organ  of  considerable 
efficiency.  The  second  and  third  methods  reach  their  highest 
development  in  mammals  and  are  best  treated  separately. 

The  interior  of  the  mammalian  nasal  cavity,  which  is  usu- 
ally very  large,  is  by  no  means  a  simple  space,  but  is  well 
filled  up  by  projecting  folds,  composed  of  thin  lamellae  of  bone 
covered  by  mucous  membrane.  These  are  termed  turtiinalia 
and  come  under  three  categories,  in  accordance  with  their 
relationships  to  other  parts:  (i)  a  naso-turbinal,  (2)  several 
ethtno-turbinals,  and  (3)  a  maxillo-turbinal. 

The  maxillo-turbinal  lies  ventral  and  usually  anterior  to 
the  others,  in  the  pars  respiratoria,  and  in  mammals  has  lost 
all  olfactory  function,  but  is  often  very  complicated  and  forms 
a  filter  or  screen  to  intercept  the  foreign  matter  in  the  air 
taken  in,  or  to  temper  it  if  cold.  This  is  the  homologue  of  the 


48o  HISTORY   OF   THE    HUMAN    BODY 

single  turbinal  found  in  certain  groups  of  reptiles,  where  its 
function  is  wholly  olfactory.  The  bone  which  forms  the 
framework  of  this  part  is  usually  distinct  from  the  ethmoid, 
and  forms  the  "  inferior  turbinated  bone  "  of  human  anatomy. 
The  name  "  maxillo-turbinal  "  is  to  be  preferred,  as  it  better 
expresses  its  relationship. 

The  remaining  turbinalia,  all  of  which  are  olfactory,  form 
a  set  of  parallel  projecting  ridges,  arising  from  the  lateral  wall 
of  the  cavity  and  arranged  in  series  from  above  downwards; 
the  most  dorsal  of  these  is  borne,  at  least  in  part,  by  the  nasal 
bone  and  is  termed  the  naso-turbinal;  the  others  are  ethnio- 
turbinalia,  that  is,  they  arise  from  the  ethmoid.  The  total 
number  of  turbinalia,  not  counting  the  maxillo-turbinal,  is 
most  usually  five,  but  larger  numbers  are  met  with  up  to  eleven, 
the  number  found  in  certain  edentates.  Besides  these  primary 
turbinalia  which,  although  arising  from  the  outer  wall,  yet 
nearly  reach  the  inner  one,  there  is  often  present  a  variable 
number  of  secondary  and  even  tertiary  turbinalia  filling  the 
spaces  left  free  by  the  first.  These  relations  are  shown  in 
Fig.  130,  A  and  B,  which  represent  diagrammatic  cross  sec- 
tions of  nasal  .cavities ;  A,  with  primary  turbinalia  alone,  and 
B,  with  secondary  and  tertiary  ones.  The  primary  ones  are 
termed  end o turbinalia  from  their  position,  to  which  all  the 
rest  are  contrasted  as  ecto turbinalia.  The  various  possibili- 
ties which  arise  from  rolling  the  edges  of  the  laminae  are  also 
shown;  the  edge  may  be  rolled  inwards  (B,  I),  or  outwards 
(B,  IV),  or  again  there  may  be  two  free  edges  either  rolled 
in  different  ways  (B,  III),  or  in  the  same  way  (B,  II'  and 
II").  This  latter  possibility,  when  viewed  from  the  free  inner 
surface,  appears  like  two  separate  turbinalia,  but  is  morpho- 
logically a  single  one. 

Such  involved  forms  of  turbinalia  are  the  rule  rather  than 
the  exception  among  quadrupedal  mammals,  the  greatest  de- 
gree of  complexity  being  reached  by  the  ungulates  (Fig.  130, 
C),  rodents  and  carnivores,  a  structure  which  gives  them  a 
high  degree  of  power  in  the  sense  of  smell.  These  structures 
become  much  reduced  in  the  anthropoids;  and  in  Man  (Fig. 


THE    SENSE-ORGANS 


481 


130,  D-F),  but  three  ethmo-turbinalia  are  usually  represented, 
and  these  are  of  the  simplest  character.  Of  these  the  two 
largest,  II  and  III,  form  the  "  upper  and  middle  concha  "  of 
human  anatomy.  The  first,  I,  is  a  rudiment,  and  the  last. 


FIG.   130.     Diagrams  of  ethmoturbinals  in  Mammals.     [After  PAULLL] 

(A)  Type  showing  endoturbinalia  alone.  (B)  Type  with  endoturbinalia  (heavy 
lines)  and  two  ranks  of  ectoturbinalia.  (C)  Diagram  of  turbinals  and  pneumatic 
cavities  in  the  ox.  (D),  (E),  (F)  Diagrams  of  three  actual  cases  in  man,  showing 
individual  variation. 


IV,  is  usually  absent.  The  "  lower  concha  "  is  a  distinct  bone, 
the  maxillo-turbinal,  and  is  not  shown  in  the  diagrams.  In  the 
human  embryo  a  larger  number  of  ethmo-turbinals  occurs, 
showing  that  man's  immediate  predecessors  possessed  a  much 


482  HISTORY    OF    THE    HUMAN    BODY 

more  highly  developed  olfactory  sense  than  appears  at  present 
(Fig.  131). 

Still  another  method  for  increasing  the  olfactory  surface  and 
thus  sharpening  the  scent  seems  to  be  found  in  the  system  of 
accessory  cavities,  hollowed  out  in  the  surrounding  bones  and 
communicating  with  one  another  and  with  the  primary  cavity. 
These  occur  in  the  maxillary,  sphenoid  and  frontal  bones,  and 
in  animals  with  the  most  elaborate  nasal  equipments  are  lined 
with  olfactory  mucous  membrane  and  may  even  develop  tur- 
binalia  themselves.  Some  of  these  cavities  are  retained  after 
the  loss  of  their  olfactory  function  and  are  lined  by  simple 
mucous  membrane.  The  largest  of  these  accessory  cavities 


FIG.  131.     Lateral  wall  of  human  nasal  cavity,  showing  the  turbinals. 

(A)    Embryo,   after  KILLIAN.      (B)    Adult,   in  part   after   WIEDERSHEIM. 
I  mx,  maxilloturbinal ;   II- VI,   ethmoturbinals. 

in  Man  is  the  sinus  maxillaris  \_antrum  of  Highmore~\  in  the 
maxillary  bone;  the  frontal  and  sphenoid  sinuses  also  belong 
to  the  same  system.  [Cf.  Fig.  130,  C-F.] 

This  extraordinary  development  of  the  organ  of  smell  in 
mammals  is  an  illustration  of  the  late  perfection  of  a  part  that 
has  existed  as  a  functional  organ  for  a  very  long  time,  yet 
without  the  necessity  of  a  high  degree  of  specialization.  The 
need  of  a  turbinal  is  first  felt  in  reptiles,  but  here  a  single  one, 
and  that  of  the  simplest  pattern,  is  found  to  suffice.  That  the 
human  nose  during  its  own  past  history  once  reached  a  much 
higher  state  from  which  it  has  since  failed  through  degenera- 
tion and  loss  of  the  parts  once  gained  is  shown  by  the  anlagen 


THE    SENSE-ORGANS  483 

in  the  embryo  of  turbinalia  that  never  develop,  and  is  indicated 
also  by  the  simple  condition  of  the  cavities,  and  the  lack  of 
complexity  in  the  turbinalia. 

An  interesting  bit  of  morphology  in  connection  with  the  his- 
tory of  the  nose  is  that  of  Jacobson's  organ  (vomero-nasal 
organ),  at  first  a  sinus  or  pocket  leading  out  of  the  main  nasal 
cavity,  later  an  independent  organ,  and  finally  a  rudiment. 
This  organ  is  first  seen  in  urodeles,  where  it  appears  upon  the 
medial  side  of  the  nasal  cavity  and  gradually  migrates  along 
the  floor,  attaining  ultimately  a  lateral  position,  though  still 
included  writhin  the  nasal  capsule.  This  migration  is  seen  by 
comparing  the  lower  with  the  higher  members  of  the  order 
and  actually  takes  place  during  the  embryological  development 
of  such  a  form  as  Triton,  where  it  may  be  followed  step  by 
step.  When  its  lateral  position  is  fully  established,  it  gradually 
restricts  its  communication  with  the  main  cavity  until  it  is 
connected  by  a  small  duct  in  the  region  of  the  posterior  nares, 
as  in  Gymnophiona.  In  Hzards  and  snakes,  where  it  reaches 
its  highest  degree  of  development,  it  forms  upon  each  side  a 
tubular,  somewhat  contorted  organ,  with  a  blind  anterior  end, 
opening  posteriorly  into  the  roof  of  the  mouth  by  an  inde- 
pendent opening,  yet  still  near  the  posterior  nares.  Its  ventral 
wall  is  rolled  up  into  its  thickened  and  strongly  convex 
dorsal  one,  and  this  latter  possesses  olfactory  sense-cells.  The 
position  of  the  organ  has  again  changed  owing  doubtless  to 
the  development  of  related  parts,  and  it  lies  almost  directly 
beneath  the  primary  nasal  cavity,  between  it  and  the  hard 
palate,  and  thus  more  nearly  in  its  original  position  near  the 
median  line. 

In  turtles,  crocodiles,  and  birds,  Jacobson's  organ  exists1 
only  in  the  form  of  embryonic  vestiges,  but,  on  the  other  hand, 
K  i&  luLdlxd  upun  cither  skle^oMhc,  carlrtegiTiOtts-iiasal  septitm^ 
a  definite  organ  and  persisting  throughout  life  in  many  cases. 

is  located  upon  either  side  of  the  cartilaginous  nasal  septum, 
and  is  protected  by  a  cartilage  of  its  own,  the  paraseptal,  vo- 
mero-nasal,  or  Jacobson's,  cartilage.  When  well  developed  the 
organ  is  in  the  form  of  a  short  tube,  which  opens  anteriorly 


484  HISTORY    OF   THE    HUMAN    BODY 

into  the  naso-palatine  canal,  thus  retaining  its  early  relation- 
ship to  the  primitive  choanse,  but  in  anthropoids  and  some 
other  mammals,  it  is  quite  vestigial  and  appears  only  in  em- 
bryonic life.  In  the  monotremes  it  reaches  the  highest  de- 
velopment attained  among  mammals,  and  is  here  entirely  en- 
cased in  its  cartilage,  through  which  passes  a  small  branch  of 
the  olfactory  nerve  to ,  supply  the  organ.  From  the  lateral 
wall  of  the  cartilage  a  turbinal  process  develops,  similar  to 
the  turbinalia  of  the  main  cavity,  but  very  simple  in  form  and 
covered  with  indifferent  non-olfactory  epithelium.  Remains 
of  this  process  are  seen  in  marsupials  and  even  in  rodents, 
forms  in  which  the  entire  organ  is  well  developed. 

If  we  except  such  special  adaptations  as  the  tubular  pro- 
longations of  the  nostrils  which  exist  in  certain  fishes  and  a 
few  aquatic  birds,  an  external  nose  as  a  separate  organ  is 
a  mammalian  characteristic.  It  possesses  a  cartilaginous  frame- 
work derived  from  the  primordial  skeleton  and  thus,  in  part, 
homologous  with  the  cartilaginous  capsule  of  amphibians,  and 
it  is  supplied  with  superficial  muscles  from  the  mimetic  group. 
It  shows  great  power  of  adaptation  in  the  various  mammals, 
sometimes  forming  a  flexible  snout  or  trunk,  as  in  swine,  ele- 
phants, and  moles,  and  sometimes  developing  a  moist,  sensitive 
surface,  as  in  carnivores  and  ruminants.  In  the  anthropoids 
it  is  reduced  in  size,  corresponding  to  the  lessened  importance 
of  the  olfactory  sense,  although  its  muscles  are  very  mobile 
and  assist  greatly  in  the  expression  of  emotions.  These  latter 
powers  seem  to  be  undergoing  degeneration  in  Man  in  spite 
of  the  fact  that  the  external  nose  is  more  prominent  than  in 
most  primates. 

The  ultimate  organ -of  smell  is  the  olfactory  membrane, 
which  in  fishes  and  amphibians  is  distributed  over  the  entire 
nasal  cavity,  but  which,  with  the  establishment  of  air-breathing 
and  the  setting  apart  of  a  respiratory  portion,  tends  to  confine 
itself  to  the  more  dorsal  region.  It  consists  of  a  highly  differ- 
entiated form  of  epithelium  in  which  occur  the  terminal  olfac- 
tory cells  surrounded  by  supporting  cells,  some  or  all  of  which 
may  be  ciliated.  In  certain  fishes  and  in  the  lower  urodeles 


THE    SEXSE-ORGAXS  485 

there  occur  in  the  nasal  mucous  membrane  definite  groupings 
of  the  olfactory  cells,  surrounded  by  protecting  cells,  thus 
forming  olfactory  buds,  almost  identical  in  form  with  the  taste- 
buds,  which  in  turn  resemble  the  lateral  line  organs.  Some  have 
seen  in  this,  as  well  as  in  the  condition  of  the  primary  olfac- 
tory pits,  which,  in  the  embryo,  form  the  anlage  of  the  nasal 
cavities,  a  possible  genetic  connection  with  the  lateral  line  or- 
gans, but  this  homology  is  rendered  improbable  from  other 
reasons.  Of  these  the  most  fundamental  is  the  singular  rela- 
tionship between  the  individual  olfactory  cells  and  their  nerve 
fibers,  the  two  being  directly  continuous,  and  not,  as  in  all 
other  known  cases  of  sensory  cells  among  vertebrates,  simply 
in  contact  with  one  another.  This  continuity  of  fiber  with  ter- 
minal cells  is,  however,  characteristic  of  the  sensory  cells  of 
the  lower  invertebrates  and  suggests  that  the  sense  of  smell, 
or  at  least  the  primary  olfactory  membrane,  has  been  inherited 
from  some  far-away  invertebrate  ancestor,  and  is  thus  much 
older  than  any  of  the  other  sense  organs.  Another  possible 
relationship  with  certain  other  parts  will  be  taken  up  below  in 
connection  with  the  lens  of  the  eye. 

The  essential  orafan  of  hearing  is  the  labyrinth  or  inner  ear, 
a  series  of  membranous  tubes  or  sacs,  the  complicated  struc- 
ture of  which  has  suggested  its  name.  In  fishes  it  is  placed 
immediately  beneath  the  bones  of  the  head  and  in  its  compara- 
tively superficial  position  needs  no  accessory  apparatus,  but 
in  the  higher  vertebrates  it  is  located  deep  in  the  interior  and 
associates  with  itself  a  number  of  auxiliary  parts  to  aid  in  the 
collection  and  transmission  of  sound  vibrations.  Probably  the 
chief  reason  for  this  difference  lies  in  the  change  from  water 
to  air,  since  the  denser  medium  transmits  the  sound  waves  with 
so  much  more  intensity  than  does  the  air  that  the  apparatus 
which  develops  in  adaptation  to  the  former  requires  an  inten- 
sifying mechanism  when  placed  in  the  latter. 

The  anlage  of  the  labyrinth  appears  in  the  early  embryo 
as  a  slight  thickening  of  the  ectoderm  over  a  small  lateral  area 
at  about  the  level  of  the  metencephalon.  As  the  cells  of  this 
area  proliferate  more  rapidly  than  those  of  the  surrounding 


486 


HISTORY   OF   THE   HUMAN    BODY 


ectoderm,  they  gradually  fold  in  and  form  a  deep  pit,  which, 
as  the  process  continues,  pushes  further  into  the  interior,  where 
it  expands  into  an  otic  vesicle,  retaining  its  connection  with  the 
exterior,  however,  through  a  narrow  tube,  the  ductus  endolym- 
phaticus.  In  selachians  this  connection  is  retained  throughout 
life,  and  a  minute  but  evident  external  pore  is  found  near  the 
top  of  the  head  which  communicates  through  a  small  duct  with 
the  interior  of  the  labyrinth ;  but  in  all  other  cases  the  connec- 
tion with  the  exterior  becomes  severed  and  the  endolymphatic 


FIG.   132.     Development  of  human  otic  capsule.     Drawn  from  model' 
by  F.  ZIEGLER,  after  WM.  His. 

duct  ends  in  a  somewhat  expanded  blind  sac,  the  saccus  endo- 
lymphaticus.  In  mammals  the  endolymphatic  duct  is  lodged 
in  a  canal  of  the  petrosal  bone,  the  aqueductus  vestibuli,  and 
enters  the  cranial  cavity,  where  its  terminal  sac  lies  just  be- 
neath (i.e.,  outside  of)  the  dura  mater.* 

*  In  some  cases  the  ductus  endolymphaticus  and  its  terminal  sac  attain 

mgh  degree  of  development  and  come  into  association  with  organs  re- 

from  its  place  of  origin.     Thus  in  certain  teleosts  the  two  ducts 

unite  into  a  median  sinus  which  is  connected  with  the  air  bladder  by  a 

cham  of  four  ossicles   (Weber's  apparatus),  developed  from  the  ribs  of 

irst  four  vertebrae.     By  this  means  the  degree  of  fullness  of  the  air- 

bladder  may  be  perceived. 

In  the  Anura  the  endolymphatic  ducts   form  a  common  sinus,   which 


THE    SENSE-ORGANS  487 

The  otic  vesicle  itself  develops,  mainly  by  the  unequal 
growth  of  the  different  regions,  into  the  labyrinth.  This  de- 
velops first  into  two  expanded  portions,  utriculus  and  sacculus, 
with  a  restricted  portion  between  them,  the  utriculo-saccular 
canal.  [Cf.  Figs.  133  and  134.]  From  the  utriculus,  which 
lies  dorsally,  develop  three  flattened  folds,  which  by  the  adhe- 
sion and  subsequent  atrophy  of  the  middle  portion  of  their 
walls,  develop  their  marginal  portions  into  tubes.  Thus  are 
formed  the  three  semicircular  canals,  which  are  constant  in 
all  vertebrates  above  the  cyclostomes,  and  vary  but  little  in 
general  appearance  or  relationships.  They  are  set  approxi- 
mately at  right  angles  to  one  another  in  such  a  way  that  one 
lies  horizontally  and  the  other  two  vertically,  but  at  an  angle 
of  about  45°  with  the  bilateral  plane  of  the  body.  They 
empty  at  either  end  into  the  utriculus,  one  end  of  each  being 
expanded  into  a  flask-shaped  ampulla.  These  are  so  placed  that 
the  ampullae  of  the  anterior  vertical  and  horizontal  canals  open 
together  into  a  pocket  of  the  utriculus  (recessus  utriculi), 
while  that  of  the  posterior  vertical  canal  opens  by  itself  into  a 
similar  pocket  on  the  other  side  (sinus  utriculi  posterior). 
The  two  vertical  canals  unite  at  their  expanded  end  and  enter 
or  form  the  sinus  utriculi  superior,  a  diverticulum  which  ex- 
tends directly  upward;  the  unexpanded  end  of  the  horizontal 
canal  enters  the  main  body  of  the  utriculus  unassociated. 

The  parts  of  the  utriculus  with  its  derivations,  the  semicir- 
cular canals,  appear  first  in  the  form  described  above  in  the 

extends  posteriorly,  dorsal  to  the  spinal  cord  and  outside  of  the  dura 
mater,  as  far  as  the  tail  rudiment  (urostyle).  This  extensive  sinus  sends 
to  the  roots  of  the  spinal  nerves  a  series  of  diverticula  which  wrap  them- 
selves about  the  spinal  ganglia  and  expand  into  sacs  containing  granules 
of  calcium  carbonate.  In  many  reptiles  the  duct  reaches  the  top  of  the 
skull  and  even  escapes,  its  terminal  sac  being  subcutaneous.  In  snakes 
this  sac  contains  calcareous  crystals,  which  in  some  cases  may  be  seen 
through  the  skin  of  the  living  animal.  This  apparatus  reaches  its  highest 
development,  so  far  as  reptiles  are  concerned,  in  the  lacertilian  family  of 
the  geckos  (Ascalabota} ,  where  the  sac  escapes  from  the  skull  through 
the  parieto-occipital  suture  and  pushes  its  way  between  the  muscles  of  the 
neck  and  shoulder  as  far  as  the  pharynx.  It  is  filled  with  a  soft  cal- 
careous mass. 


488  HISTORY   OF   THE    HUMAN    BODY 

selachians  and  are  retained  with  very  little  deviation  by  all 
higher  vertebrates.  The  sacculus,  however,  exists  in  a  simple 
form  in  fishes  and  shows  considerable  advance  in  the  higher 
forms.  This  advance  consists  of  the  gradual  development  of 
a  lateral  sac,  the  lageyia,  which  is  situated  upon  the  inner  side 
and  which  in  fishes  and  amphibians  is  barely  indicated.  In 
reptiles  and  birds  the  lagena  becomes  considerably  elongated 
and  curved,  and  in  mammals  it  becomes  spirally  wound  and, 
associating  certain  outside  elements  with  itself,  forms  the 
cochlea,  here  attaining  the  highest  degree  of  complexity  of 
any  part  of  the  labyrinth.  In  the  higher  forms  also  the  direct 
connection  between  utriculus  and  sacculus  becomes  replaced 
by  an  indirect  one  through  the  ductus  endolymphaticus,  which 
arises  by  two  branches,  one  from  each  of  the  two  parts  [Cf. 
Fig.  134].  These  unite  and  thus  indirectly  retain  the  con- 
nection in  question. 

The  labyrinth  of  the  cyclostomes  stops  at  a  lower  point  of 
development  than  is  represented  by  any  of  the  gnathostomes 
and  may  well  represent  the  permanence  of  what  is,  in  the 
higher  forms,  an  early  embryonic  stage.  It  consists  of  a  simple 
oval  sac,  not  yet  differentiated  into  utriculus  and  sacculus,  and 
possessed  of  either  one  (Myxine)  or  two  (Petromyzori)  semi- 
circular canals.  Its  endolymphatic  duct,  however,  is  short  and 
loses  its  connection  with  the  exterior  (Fig.  133,  a). 

The  walls  of  the  embryonic  labyrinth  are  composed  of  a 
single  layer  of  epithelial  cells  of  appreciable  thickness  and  all 
alike;  as  development  proceeds,  however,  the  greater  part  of 
the  cells  become  flattened  and  form  a  transparent  membrane, 
while  over  certain  definite  areas,  6  to  8  in  all,  the  cells  are 
thickened  and  columnar,  forming  neuro-epithelinm.  Certain 
of  these  cells  form  the  ultimate  organs  of  hearing  and  are 
provided  with  various  sorts  of  terminal  flagella  and  other 
similar  structures  (auditory  hairs),  which  project  into  the  lu- 
men of  the  labyrinth  and  are  bathed  in  the  endolymph,  i.e.,  the 
fluid  filling  the  interior.  These  are  supplied  with  nerve  fibers 
from  the  auditory  nerve.  About  these  are  placed  various  sorts 
of  supporting  cells,  which  are  without  auditory  function  and 


THE    SENSE-ORGANS 


489 


serve  to  hold  and  protect  the  others.  These  spots,  which  thus 
show  a  higher  degree  of  cellular  differentiation,  are  the  seat 
of  the  auditory  sense,  and  appear  from  their  greater  thickness 


a 


Mac 


Hacsac  Pap  lagKoch) 

FIG.  133.     Ear  labyrinths  of  various  Vertebrates.     [After  RETZIUS.] 

(a)  Cyclostome;  Myxine  glutinosa  (hag  fish),  (b)  Selachian;  Chimaera  monstrosa, 
(c)  Teleost;  Anarrhichas  lupus  (wolf-fish),  (d)  Amphibian;  Rana  esculenta  (frog), 
(e)  Bird;  Bubo  ignavus  (horned  owl),  (f)  Mammal;  Sus  scrofa  domestica  (pig). 
aa,  ampulla  anterior;  ae,  ampulla  extcrna;  ap,  ampulla  posterior;  e,  ductus 
endolymphaticus;  fee,  facial  nerve;  mac,  macula  communis;  mac.  ittr,  macula 
utriculi;  mac.  sac,  macula  sacculi;  n,  macula  neglecta;  pap.  lag,  papilla  lagenae; 
pap.  bos,  papilla  basilaris. 

as  white  patches  in  the  otherwise  transparent  wall.    They  are 
further  indicated  by  their  association  with  the  nerve. 

The  largest  occurring  number  of  such  auditory  areas  is  eight, 
although  all  do  not  occur  simultaneously.  Of  these  the  three 
which  are  associated  with  the  semicircular  canals  are  some- 


490  HISTORY    OF   THE    HUMAN    BODY 

what  different  from  the  rest  and  are  absolutely  constant.  They 
are  situated  in  the  ampullae  and  are  in  the  form  of  ridges  which 
encircle  them  and  project  into  the  lumen.  They  are  thus  dis- 
tinguished from  the  others  as  cristcz  acusticcz,  The  remaining 
five  are  evidently  formed  by  the  successive  breaking  up  of  a 
single  large  area,  due  to  the  differentiation  of  parts  of  the  laby- 
rinth, and  the  history  of  this  segmentation  and  later  migration 
is  represented  in  the  phylogenetic  series  (Fig.  133).  Thus,  in 
the  cyclostomes  the  acoustic  region,  aside  from  the  one  or  two 
semicircular  canals,  is  represented  by  a  single  area,  macula 
acustica  communis,  covering  the  bottom  of  what  is  here  a 
simple  sac  (Fig.  133,  a).  The  differentiation  of  utriculus  and 
sacculus  gives  a  separate  area  to  each,  respectively,  the  ma- 
cula acustica  recessus  utriculi  and  macula  acustica  sacculi 
(Fig.  133,  b).  With  the  gradual  development  of  the  lagena 
in  fishes  there  appears  an  outgrowth  of  this  latter  area  which 
finally  separates  from  its  place  of  origin  and  establishes  itself 
as  the  auditory  area  for  the  newly  developed  part,  under  the 
name  of  papilla  acustica  lagena.  This  gradual  separation  of 
both  lagena  and  its  acoustic  area  is  accompanied  by  a  similar 
separation  of  the  nerve,  which  splits  off  a  supply  branch  for 
the  new  area.  (Fig.  133,  cf.  b  and  c.)  A  small  macula  is 
also  formed  near  the  utricular  macula,  the  macula  acustica  neg- 
lecta,  evidently  an  offshoot  from  the  latter,  although  no  phylo- 
genetic proof  of  this  appears  as  in  the  former  case.  The  fifth 
and  last  of  the  auditory  areas,  not  counting  the  cristse  acus- 
ticae  of  the  ampullae,  the  papilla  acustica  basilaris,  also  be- 
longs in  the  lagenar  region,  and  appears  in  the  higher  urodeles 
as  an  offshoot  of  the  papilla  lagenae  (Fig.  133,  d). 

To  put  these  points  into  the  form  of  a  phylogenetic 
history  we  may  take  as  a  starting  point  the  macula  communis 
of  cyclostomes,  which  may  be  considered  to  hold  all  the  later 
elements  within  itself.  In  fishes  we  find  a  primary  macula  for 
each  of  the  two  parts  into  which  the  labyrinth  has  become 
divided,  also  a  macula  neglecta,  which  has  presumably  sepa- 
rated from  the  one  belonging  to  the  utriculus  at  some  point 
below  the  fishes.  Within  the  group  of  the  selachians  the 


THE    SENSE-ORGANS  491 

papilla  lagenae  appears,  sometimes  as  a  separate  area,  some- 
times as  a  lobe  of  the  macula  sacculi,  thus  giving  four  acoustic 
maculae  for  most  fish.  In  amphibians  the  papilla  basilaris  ap- 
pears while  the  others  are  retained,  although  there  is  a  slight 
transposition  of  the  macula  neglecta.  This  condition,  with 
five  acoustic  areas,  is  retained  by  the  Sauropsida  with  some 
variation,  such  as  the  division  of  the  papilla  basilaris  in  certain 
lizards  and  a  great  reduction  of  the  macula  sacculi  in  turtles, 
points  referable  to  special  adaptation  and  of  no  general  signifi- 
cance. In  mammals  there  are  important  changes.  The 
macula  neglecta  has  entirely  disappeared  and  the  papilla  lagena 
is  found  only  in  Ornithorhynchus  (monotreme),  leaving  in 
this  Class  but  three  acoustic  areas  aside  from  the  three  cristae 
acusticse  of  the  semicircular  canals. 

The  most  important  difference  in  the  mammalian  labyrinth 
is  the  great  development  of  the  lagena.  The  tendency,  already 
shown  in  crocodiles  and  birds,  to  prolong  this  part  and  to  curve 
its  axis,  results  here  in  an  excessive  elongation  which  becomes 
wound  into  a  close  spiral,  the  nerve  forming  the  central  axis. 
The  number  of  complete  coils  in  man  is  about  3,  but  varies 
among  mammals  between  the  limits  of  ij  and  5.*  This 
coiled  lagena  becomes  complicated  by  the  addition  of  parts  of 
the  outer  bony  labyrinth,  to  be  explained  later,  which  form  two 
additional  coiled  passages,  scala  vestibuli  and  scali  tympani, 
that  receive  between  them  the  lagena  under  the  anatomical 
name  of  ductus  cochlearis  [scala  media'}.  This  entire  organ, 
including  both  this  part  of  the  labyrinth  and  its  accessory  or- 
gans, is  called  the  cochlea. 

The  papilla  basilaris  lies  along  the  floor  of  the  coiled  lagena 
(scala  media)  and  becomes  highly  differentiated  into  a  number 

*  Examples  are  as  follows : 

Erinaceus    (Hedgehog)     i^ 

Whales  and  porpoises 1^2 

Rabbit   2^ 

Cat    , 3 

Ox    354 

Swine 3l/2 

Ccelogenys    (South  American   rodent) 5 


492 


HISTORY   OF    THE    HUMAN    BODY 


of  kinds  of  histological  elements,  which  change  their  propor- 
tionate size  along  the  course,  being  the  smallest  at  the  base  and 
the  largest  at  the  apex.  The  most  important  of  these  cellular 
elements  are  certain  elongated  cells  associated  in  pairs,  the 
rods  of  Corti,  and  two  groups  of  cells  with  specialized  terminal 
organs,  the  outer  and  inner  hair  cells.  The  ventral  portion  of 
the  lagena,  that  is,  the  floor  of  the  scala  media  beyond  the 
auditory  area,  is  termed  the  basilar  membrane,  and  the  dorsal 
portion  (roof)  is  Reissner's  membrane;  these  terms  are,  how- 
ever, purely  anatomical  ones,  expressing  certain  relationships 


canalis  cochlearis. 


FIG.  134.  Diagram  of  membranous  labyrinth  of  human  ear.  [From 
GEGENBAUR,  after  RETZIUS.] 

•    A,    A,    A,    ampullae;     U,    utriculus;    S,    sacculus;     E,    ductus    endolymphaticus; 
ant,  ext,  post,  the  three  semicircular  canals. 

to    the   surrounding  parts    and   are   without   morphological 
significance. 

The  membranous  labyrinth  as  above  described,  that  is,  the 
higher  development  of  the  auditory  vesicle  of  the  embryo,  be- 
comes surrounded  while  still  embryonic  by  a  gelatinous  connec- 
tive tissue,  its  first  accessory  organ.  Later  on  in  development 
this  tissue  becomes  converted  either  to  cartilage  or  bone,  leav- 
ing, however,  a  nearly  uniform  layer  of  the  original  tissue  be- 
tween it  and  the  membrane.  There  is  thus  formed  a  mold 
which  reproduces  the  membranous  labyrinth  in  its  details,  the 
bony  (or  cartilaginous)  labyrinth.  The  gelatinous  tissue  be- 
comes soon  converted  into  a  serous  fluid,  called  the  perilymph, 
in  distinction  from  the  endolymph  of  the  interior,  and  the  cav- 
ities involved  are  conveniently  distinguished  as  the  perilym- 


THE    SENSE-ORGANS  493 

phatic  and  endolymphatic  cavities  respectively.  The  mem- 
branous labyrinth  is  held  in  place  by  scattered  strings  of  the 
original  connective  tissue,  which  connects  it  with  the  Sony  wall, 
aside  from  which  the  two  come  into  close  contact  at  the  places 
of  entrance  of  the  various  branches  of  the  auditory  nerve.  In 
the  lower  vertebrates  the  outer  labyrinth  remains  as  a  mold 
imbedded  in  the  petrosal  bone,  but  in  higher  forms,  especially 
in  mammals,  the  bony  labyrinth  appears  over  certain  regions, 
especially  the  semicircular  canals  and  the  lagena,  as  a  thin  but 
very  hard  wall,  with  a  space  between  its  outer  surface  and  the 
main  mass,  thus  reproducing  from  without  also,  the  main  de- 
tails of  the  membranous  labyrinth. 

The  transition  from  water  to  air,  undoubtedly  the  greatest 
change  which  vertebrates  have  ever  experienced,  and  one  which 
demanded  modifications  affecting  every  part,  affected  the  organ 
of  hearing  directly,  for  a  change  was  made  from  a  denser 
medium,  which  readily  transmitted  sound  vibration,  to  a 
lighter  one  in  which  transmission  was  more  difficult.  This 
disadvantage  was  undoubtedly  felt  by  the  urodeles,  which  ex- 
hibit a  new  organ,  evidently  destined  to  assist  in  the  reception 
of  less  powerful  vibrations.  In  the  cartilaginous  otic  capsule 
surrounding  the  labyrinth,  that  which  partly  corresponds  to  the 
"  bony  labyrinth  "  of  higher  forms,  there  exists  an  oval  opening 
with  a  reinforced  rim,  closed  byx  an  ossicle  in  the  form  of  a  lid, 
usually  with  a  process  projecting  from  its  center  [cf.  Fig.  39, 
op~\.  The  opening,  which  persists  in  all  higher  vertebrates,  is 
the  fenestra  ovalis,  and  the  osseous  lid,  which  is,  in  origin,  a 
portion  cut  off  from  the  wall  of  the  capsule,  is  the  operculum. 
This  latter  is  fitted  to  the  rim  of  the  fenestra  ovalis  by  a 
membrane,  and,  as  it  is  nearly  subcutaneous,  it  is  set  in  mo- 
tion by  the  impact  of  sound  waves,  and  thus  serves  to  slightly 
intensify  the  vibrations. 

This  apparatus  proves  sufficient  for  urodeles,  which  are 
much  in  the  water,  but  in  the  tailless  forms  (Anura),  far  more 
terrestrial  than  the  salamanders,  the  sound-receiving  apparatus 
is  much  improved  by  an  important  addition,  the  tympanum,  or 
cavity  of  the  middle  ear.  This  is  developed  from  the  gill 


494  HISTORY   OF   THE   HUMAN    BODY 

pouch  of  the  spiracular  opening,  the  one  associated,  as  will  be 
remembered,  with  the  hyoid  arch.  The  inner  portion  of  this 
cavity  communicates  with  the  pharynx  and  forms  the  auditory 
or  Eustachian  tube,  but  direct  communication  with  the  outside 
is  prevented  by  the  presence  of  a  circular  tympanic  membrane 
at  the  outer  end,  just  beneath  the  skin,  and  usually  very  evi- 
dent from  the  outside. 

This  membrane,  which  is  covered  outwardly  by  integument 
and  on  its  inner  side  by  mucous  membrane,  is  a  separate 
formation,  usually  of  connective  tissue,  but  in  a  few  cases  it  is 
cartilaginous.  It  has  been  doubtfully  homologized  with  the 
spiracular  cartilage  of  selachians,  but  this  is  too  uncertain  to 
be  definitely  asserted.  In  many  cases  there  exists  a  second 
opening  in  the  wall  of  the  otic  capsule,  the  fenestra  cochlea? 
[rotunda],  filled  with  a  thin  membrane,  also  termed  the  inner 
tympanic  membrane.  This  part  is  present  in  all  higher  verte- 
brates, thus  giving  the  tympanum  the  characteristic  from 
which  it  derives  its  name,  i.e.,  two  drum  heads,  outer  and 
inner.  To  complete  the  likeness  of  the  middle  ear  to  a  drum 
the  Eustachian  tube  represents  the  opening  always  present  in 
the  cylinder  of  a  drum  and  employed  in  both  purposes  for 
equalizing  the  air  pressure  on  either  side  of  the  drum  heads. 

A  mechanism,  however,  which  is  lacking  in  a  drum,  is  that 
formed  by  the  columella,  a  delicate  spindle  of  bone  or  cartilage, 
which  extends  from  the  center  of  the  outer  tympanic  mem- 
brane to  the  operculum.  By  this  means  the  sound  vibrations 
that  impinge  upon  the  former  are  transmitted  directly  to  the 
latter,  and  through  it  to  the  perilymph  within  the  otic  capsule. 
Another  channel  for  the  transmission  of  sound  waves  is  fur- 
nished by  the  air  enclosed  in  the  tympanic  cavity,  the  vibrations 
striking  the  inner  tympanic  membrane.  The  apparently  new 
skeletal  element,  the  columella,  is  probably  nothing  more  than 
a  process  of  the  operculum,  but  it  has  been  considered  by  some 
to  be  a  distinct  element  and  to  represent  the  hyo-mandibular 
of  fishes,  employed  there  as  a  suspensory  piece  for  the  man- 
dible and  originally  the  dorsal  segment  of  the  second  visceral 
arch  (hyoid). 


THE    SENSE-ORGANS  495 

In  the  Sauropsida  there  is  but  little  change  in  the  tympanic 
cavity  from  that  of  the  Anura.  The  two  Eustachian  tubes 
often  form  by  their  union  a  median  duct,  which  opens  into  the 
pharynx  in  the  mid-dorsal  line.  Such  is  the  case  in  birds  and 
in  crocodiles,  and  in  the  latter  the  tubes  under  consideration 
form  a  complicated  system  of  cavities,  many  of  which  are 
lodged  in  the  bones  of  the  cranium.  In  other  cases  similar  sys- 
tems extend  out  from  the  main  tympanic  cavity,  and  in  certain 
instances  the  two  latter  communicate  with  one  another  across 
the  median  line. 

A  characteristic  and  important  addition  is  gained  in  mam- 
mals by  the  appearance  within  the  tympanic  cavity  of  the  artic- 
ular and  quadrate  bones,  hitherto  employed  in  forming  the 
mandibular  articulation.  These  form  respectively  the  malleus 
and  incus,  and  become  added  to  the  columella  to  form  a  chain 
of  ossicles  which  reaches  across  the  cavity  from  the  other  drum 
head  to  the  fenestra  ovalis,  thus  assuming  the  function  formerly 
sustained  by  the  columella  alone.  This  latter  apparently  be- 
comes reduced  in  size  and  forms  the  stapes.  (Cf.  Chap.  V.) 
The  singular  and  characteristic  foramen  in  this  bone,  to  which 
it  owes  its  similarity  to  a  stirrup,  is  caused  by  the  development 
of  a  small  artery,  which  perforates  the  columella.  This  re- 
lation appears  only  in  the  embryo  in  most  mammals,  includ- 
ing Man,  but  in  some  (certain  rodents  and  insectivores)  it  per- 
sists throughout  life.  (Cf.  Chap.  IX.)  In  others  still,  mainly 
monotremes  and  marsupials,  the  perforation  does  not  take 
place,  but  the  bone  remains  in  the  primitive  cylindrical  form. 

The  stapes  is  supplied  by  a  tiny  muscle,  the  stapedius,  which 
is  shown  by  its  embryology  to  be  a  slip  separated  from  the 
digastric  muscle,  an  element  primarily  associated  with  the 
hyoid  arch.  To  the  malleus  is  attached  a  second  small  muscle, 
the  tensor  tympani.  This  was  originally  a  portion  of  the  com- 
mon mass  from  which  the  masticatory  muscles  of  the  jaw  have 
differentiated,  the  abductor  mandibuli  of  the  selachians.  This 
little  slip  arises  from  that  portion  which  forms  the  pterygoid 
muscles,  and  is  innerved  by  a  branch  from  Trigeminus,  the 
nerve  associated  with  the  first  or  mandibular  arch.  These  two 


496  HISTORY    OF    THE    HUMAN    BODY 

tympanic  muscles  have  thus  had  a  history  as  old  as  the  parts 
to  which  they  are  attached,  and  form  here,  together  with 
their  associated  nerves  and  ossicles,  groups  of  parts  which 
have  retained  their  original  relationships  through  all  their 
migrations  and  changes  of  form  and  function. 

Another  characteristic  mammalian  element,  not  directly 
within  the  tympanic  cavity  but  closely  associated  with  it,  is 
the  tympanic  bone  (os  tympanicum).  This,  when  in  its  full 
development,  forms  a  complete  bony  ring  or  frame  about  the 
outer  tympanic  membrane,  and  often  develops  in  addition  a 
concave  osseous  shell  or  tympanic  bulla,  which  forms  a  conspic- 
uous object  at  the  base  of  the  skull  and  aids  in  protecting  the 
delicate  parts  of  the  middle  ear.  Occasionally,  too,  the  bone 
extends  outwards  to  form  an  osseous  wall  for  the  external 
meatus.  This  bone  remains  distinct  throughout  life  in  mono- 
tremes,  marsupials  and  a  few  others,  but  in  the  majority  of 
cases,  as  in  Man,  it  fuses  with  the  petrous  elements  and  becomes 
eventually  lost  in  the  complex  designated  as  the  "  temporal 
bone."  The  homologies  of  this  bone  are  uncertain,  although 
some  consider  it  the  same  as  sauropsidan  quadrato-jugal.  It 
can  hardly  be  a  new  osseous  element,  but  that  it  appears  here 
in  a  new  role  and  is  thus  a  new  bone  physiologically  is  evident. 

The  external  ear,  characteristic  of  the  mammalia,  is  mainly 
a  cartilaginous  structure  covered  by  integument,  and  consists 
of  a  round  tube,  the  external  auditory  meatus,  and  an  external 
flap,  the  auricula  [pinna].  The  first  of  these,  the  meatus, 
allows  the  outer  tympanic  membrane  to  sink  below  the  surface 
and  still  retain  connection  with  the  exterior  and  its  curve 
affords  the  membrane  a  more  or  less  complete  concealment. 
In  cases  where  the  tympanic  bone  furnishes  a  prolonged  tube 
for  this  purpose,  the  external  cartilaginous  element  is  less  ex- 
tensive, and  the  two  together  form  the  wall  of  the  canal. 

The  auricula  shows  a  large  degree  of  adaptation,  being  very 
large  and  mobile  in  cases  where  acute  hearing  is  desired,  for 
example,  in  bats,  and  in  most  ungulates;  and  is  reduced  or 
entirely  wanting  in  many  burrowing  or  aquatic  forms.  The 
characteristic  anthropoid  ear  is  shaped  at  the  base  much  as  in 


THE    SENSE-ORGANS  497 

Man,  but  there  is  no  lobule  and  little  or  no  reclining  to  the  free 
edge.  This  latter  peculiarity,  which  starts  at  the  upper  part  of 
the  base,  is  distinctively  human,  but  extends  over  a  varying  dis- 
tance in  different  individuals,  and  is  often  hardly  begun  in  the 
new-born  infant.  A  rudimentary  point,  tuberculum  auriculi 
[Darwini],  is.  often  retained  at  the  free  edge,  and  is  brought 
over  by  the  rolling  process  so  as  to  point  forward  instead  of 
backward,  its  primary  position.  This  is  occasionally  a  con- 
spicuous feature,  and  in  all  cases  its  place  can  be  determined 
by  feeling,  being  indicated  by  a  thicker,  harder  area  on  the 
outer  rim  a  little  below  the  top  of  the  curve.  A  lobule  is 
usually  present,  but  is  rudimentary  or  absent  in  certain  races. 

Concerning  the  origin  of  the  cartilaginous  elements  of  the 
external  ear,  it  becomes  evident  from  the  condition  found  in 
monotremes  that  it  is  largely  or  wholly  derived  from  the  upper 
end  of  the  hyoid  arch,  which  curves  about  the  tympanic  mem- 
brane and  forms  a  tubular  meatus  together  with  a  rudimentary 
pinna.  This  leaves  unaccounted  for  a  series  of  protuberances 
in  the  integument  surrounding  the  opening  of  the  meatus, 
which  are  seen  to  form  in  the  human  embryo  and  fuse  to- 
gether to  build  up  the  external  portion  (pinna).  These  pro- 
tuberances are  considered  by  some  to  be  elements  furnished 
by  the  first  four  visceral  arches,  i.e.,  mandibular,  hyoid  and 
the  first  two  branchial,  but  this  is  rendered  very  improbable 
by  the  innervation  of  the  pinna,  which  is  wholly  from  the 
Facialis.  It  may  thus  prove  to  be  a  modification  of  devel- 
opment, and  portions  which  were  originally  hyoid  elements  may 
here  appear  in  this  form.  In  the  Sauropsida  the  outer  tym- 
panic membrane  is  frequently  depressed  a  little  below  the  sur- 
face and  provided  with  small  protuberances  or  flaps  which  as- 
sist in  its  protection,  but  these  are  evidently  incidental  adapta- 
tions and  can  have  nothing  to  do  with  the  external  ear  of  mam- 
mals. 

The  developmental  history  of  the  eye,  as  given  in  the  pre- 
vious chapter,  shows  that  this  sense-organ,  that  is,  its  essential 
part,  the  retina,  differs  radically  from  all  the  others  in  being 
originally  a  portion  of  the  brain  surface,  the  cells  of  which  have 


498  HISTORY    OF    THE    HUMAN    BODY 

become  specialised  in  form  and  function  so  that  they  respond 
directly  to  the  stimulus  of  light  vibrations.  To  this  essential 
part,  which,  with  a  pigmented  outer  layer,  are  the  only  parts 
derived  from  the  brain,  accessory  organs  are  added  from  two 
sources  to  complete  the  formation  of  the  eyeball;  the  lens, 
formed  from  the  ectoderm  of  the  outer  surface;  the  chorioid 
and  sclerotic  coats  and  the  vitreous  body  from  the  surrounding 
connective  tissue.  Aside  from  the  eyeball  itself  there  are 
many  external  accessory  parts,  such  as  muscles  and  glands, 
conjunctiva  and  eyelids,  which  come  from  several  sources  and 
aid  in  the  movement  and  protection  of  the  sensitive  organ. 

To  begin  with  the  essential  sense-organ,  that  is,  the  retina,  if 
we  follow  the  in-  and  outpushings  of  its  layer  of  origin  from 
the  beginning,  it  is  clear  that  the  original  external  surface  lines 
the  lumen  of  the  neural  tube  and  eventually  forms  the  outer 
retinal  surface,  that  is,  the  surface  turned  toward  the  pigmented 
tapetum.  Now  in  all  cases  it  is  the  primarily  external  surface 
that  becomes  specialized  to  receive  external  stimuli,  and  it  is 
also  the  original  outer  or  superficial  end  of  the  sensory  cells 
that  develop  the  specially  modified  flagella  and  other  parts.  It 
thus  happens  that  the  terminal  cells  of  the  sense  of  vision  are 
not  only  the  outer  ones  of  the  retina,  which  is  several  cell- 
layers  in  thickness,  but  also  that  their  free  ends,  bearing  the 
terminal  rods  and  cones,  point  in  the  same  direction,  namely, 
towards  the  interior  of  the  head  and  away  from  the  source  of 
light.  Moreover,  since  a  sensory  nerve  must  approach  its 
terminal  cells  from  their  physiological  inner  side,  this  arrange- 
ment compels  the  optic  nerve  first  to  penetrate  the  entire  retina 
and  attain  the  interior  of  the  eyeball,  and  there  spread  out  its 
separate  fibers,  which  severally  become  recurved  and  pass  back 
again  through  the  retina  to  supply  the  terminal  cells.  Finally, 
in  order  that  the  image,  received  through  the  pupil  and  focused 
by  the  lens  upon  the  physiological  outer  side  of  the  retina,  may 
reach  the  terminal  rods  and  cones,  all  the  intervening  parts,  the 
nerve  fibers  and  the  various  layers  of  retinal  cells,  have  to  be 
perfectly  transparent;  and,  furthemore,  the  terminal  rods  and 
cones  must  needs  be  buried  in  the  pigment  of  the  tapetum  in 


THE    SENSE-ORGANS  499 

order  to  stop  possible  light  impressions  from  coming  from  with- 
out the  eyeball,  i.e.,  the  natural  direction  for  the  receptive 
cells. 

The  necessity  of  this  arrangement  becomes  clear  to  anyone 
who  has  followed  the  foldings  of  the  embryonic  layers,  yet 
there  is  scarcely  anything  in  vertebrate  construction  that  seems 
a  greater  mechanical  mistake,  although  there  are  many  others, 
like  the  appendix  and  the  inguinal  canal  in  man,  where  a  lesser 
error  involves  far  more  serious  consequences.  This  error  in 
the  arrangement  of  the  retina,  however,  becomes  still  more  ap- 
parent when  a  comparison  is  made  with  the  structurally  sim- 
ilar eye  of  the  cephalopod  molluscs  (squid,  devil-fish,  etc.),  in 
which  the  retina  is  developed  directly  from  the  surface  ecto- 
derm and  is  placed  in  the  natural  way,  with  the  terminal  cells 
lining  its  interior  and  the  optic  nerve  entering  it  from  behind. 
Notwithstanding  the  fundamental  differences  in  developrhent 
between  this  eye  and  that  of  vertebrates,  the  final  results,  when 
compared  part  by  part,  are  marvelously  similar,  and  the  adult 
eye  of  each  is  furnished  with  retina  and  crystalline  lens,  iris 
and  cornea.  This  case  is  thus  one  of  the  best  examples  of  what 
Mr.  Darwin  termed  "  analogical  resemblances  " ;  that  is,  the 
production  of  a  similar  organ  in  two  unrelated  forms  and  often 
from  entirely  different  starting  points,  not  through  any  genetic 
connection,  but  because  of  the  same  environmental  influences, 
which  give  rise  to  the  same  necessities. 

In  its  histological  structure  the  vertebrate  retina  shows  some 
similarity  to  other  well-developed  portions  of  the  brain,  and  ex- 
hibits several  layers  of  cells,  connected  with  one  another  by 
branching  processes  which  interlace  and  thus  continue  the  com- 
munication from  one  to  another.  At  the  exact  focal  center  of 
the  lens  all  but  the  terminal  sense-cells  disappear,  and  produce 
a  small  depressed  area,  the  area  centralis.  This  is  often  in  the 
form  of  a  circular  pit,  fovea  centralis,  but  may  be  oval,  or  in 
the  form  of  a  broad  band  or  streak.  It  is,  however,  not  always 
depressed,  and .  may  be  entirely  wanting.  These  variations 
seem  to  bear  little  or  no  relation  to  phylogeny,  since  a  fovea  is 
present  in  some  fishes  and  in  most  Sauropsida,  while  the  area 


500 


HISTORY   OF   THE    HUMAN    BODY 


seems  entirely  lacking  in  many  mammals  (Insectivora  and 
some  rodents).  In  the  anthropoids  it  is  very  pronounced,  and 
in  man  it  is  designated  by  a  yellow  color  (hence  "macula 
lutea").  Certain  birds  possess  two  such  areas,  medial  and 
lateral. 

To  understand  the  addition  of  the  accessory  organs  and  the 
formation  of  the  eyeball  it  is  necessary  to  examine  more 
thoroughly  the  early  stages  in  the  formation  of  the  optic  cup. 
The  study  of  a  few  actual  sections  will  show  that  the  invagina- 

a  h 


FIG.   135.     Diagrams  of  the  retina. 


^  (a)  Section  including  the  fovea,  showing  the  separate  elements.  [From 
GEGENBAUR,  after  RAMON  v  CAJAL.]  (b)  More  conventionalized  representation  of 
retinal  layers.  [After  GEGENBAUR.] 

fov  fovea;  /,  membrana  limitans  interna;  II,  nerve  fiber  layer;  III,  nerve  cell 
layer;  IV  inner  granular  layer;  V,  inner  nuclear  layer;  VI,  outer  granular  layer; 
VII,  outer  nuclear  layer;  VIII,  membrana  limitans  externa;  IX,  rod  and  cone 
layer;  X,  tapetum. 

tion  of  the  primary  outpushing  is  not  a  symmetrical  one,  but  is 
so  effected  that  the  cup  is  deficient  for  a  little  space  on  the  ven- 
tral aspect,  and  that  this  deficiency  is  continued  as  a  groove 
along  the  lower  side  of  the  optic  stalk.  It  thus  happens  that 
when  the  lens,  which  at  this  time  is  added  to  the  optic  cup,  be- 
comes closely  applied  to  its  rim,  a  fissure  or  oblong  aperture,  the 
chorioid  fissure,  is  left,  through  which  communication  may  be 
made  with  the  interior  of  the  cup  behind  the  lens.  Through 
this  inlet  migrate  embryonal  connective  tissue  cells  (mesen- 


THE    SENSE-ORGANS 


501 


chyma)  and  form  a  gelatinous  tissue,  the  basis  of  the  vitreous 
humor.  From  similar  mesenchymatous  elements  added  exter- 
nally is  formed  the  vascular  network  of  the  chorioid  coat,  and 
outside  of  this  is  formed  the  solera  [sclerotic  coat].  The  an- 
terior portion  of  the  chorioid  forms  the  iris  and  the  corre- 
sponding portion  of  the  sclera  forms  the  cornea.  This  latter 
stands  out  from  the  lens  in  front  and  thus  forms  an  anterior 
chamber,  filled  with  the  aqueous  humor,  a  colorless  lymph, 
which  serves  as  a  refracting  medium.  The  corresponding 
chamber  of  the  cephalopod  eye  is  perforated  by  a  foramen 
communicating  with  the  exterior,  and  through  this  it  is  filled 
with  sea  water  which  serves  the  same  purpose.  This  expe- 


FIG.   136.     Development  of  the  optic  cup. 

(a)    Plastic  representation.      [After  HERTWIG.]      (b)    Median  longitudinal  section  of 
(a),      (c)   Cross  section  in  plane  indicated  by  the  line  xy. 

dient  is  comparable  with  that  of  the  internal  ear  of  selachians, 
with  its  direct  communication  with  the  exterior  through  the 
ductus  endolymphaticus. 

The  crystalline  lens,  the  formation  of  which  has  been  alluded 
to  elsewhere,  is  a  product  of  the  ectoderm  and  appears  first  as  a 
thickening  opposite  the  optic  cup.  It  soon  invaginates  and 
pinches  off  from  its  layer  of  origin,  at  first  as  a  vesicle  with  a 
nearly  uniform  wall  and  a  central  lumen.  The  posterior  wall 
soon  thickens  and  restricts  the  lumen  more  and  more  until  this 
latter  becomes  entirely  suppressed,  while  the  wall  itself,  becom- 
ing lenticular  in  shape,  is  covered  by  the  anterior  portion  as 
by  a  cap.  This  thickening  is  produced  by  an  extreme  elonga- 
tion of  the  cells,  which  remain  in  the  form  of  a  single  layer. 


502  HISTORY   OF   THE   HUMAN    BODY 

The  functional  lens  is  formed  by  the  cornification  of  these  cells, 
and  the  mass  thus  formed  is  covered  anteriorly  with  a  thin 
epithelium,  the  original  anterior  wall  of  the  vesicle. 

It  will  be  noticed  that  there  is  in  this  development  of  the 
lens  a  striking  similarity  with  the  early  stages  of  both  the  nose 
and  the  ear,  and  if  there  be  taken  in  connection  with  these  cer- 
tain temporarily  thickened  areas  of  the  external  ectoderm  in 
association  with  the  Facialis  and  the  Glosso-pharyngeus 
nerves,  which  appear  and  vanish  again  during  the  embryonic 
life  of  the  lower  vertebrates,  the  idea  comes  at  once  to  mind 
that  we  have  here  the  record  of  a  series  of  ancient  sense-organs 
laterally  placed,  perhaps  a  pair  for  each  metamere,  some  of 
which  have  specialized  in  various  ways  while  others  have  be- 
come lost.  If  this  be  true,  the  lens  was  originally,  not  a  re- 
fracting medium,  but  a  sense-organ  itself,  which  has  given  up 
its  primary  function  entirely  and  entered  the  service  of  another 
sense-organ,  different  in  origin  from  that  of  any  other  in  ver- 
tebrate history,  namely,  a  specialization  of  a  portion  of  brain 
surface. 

The  idea  of  this  ancient  series  of  sense-organs  suggests 
many  questions.  What  was  the  primary  function  of  this 
series?  Did  these  sense-organs  sustain  any  relation  to  trie 
lateral  line  organs  ?  To  these  questions,  belonging  themselves 
to  the  realm  of  pure  suggestion,  we  can  give  but  speculative 
answers.  Both  the  nose  and  the  ear,  as  we  have  already  seen, 
have  in  their  structure  and  development  something  to  suggest 
a  kinship  with  the  lateral  line  organs ;  this  is  especially  true  of 
the  latter,  with  its  nerve  appearing  in  connection  with  that  ele- 
ment of  the  facial  nerve  that  supplies  these  organs  in  fishes, 
and  with  its  semicircular  canals  that  resemble  the  canals  of 
Lorenzini.  The  eye  itself  cannot,  of  course,  be  included  in  any 
series  of  sense-organs  of  integumental  origin,  but  the  lens  can, 
and  there  seems  no  intrinsic  difference,  up  to  a  certain  stage, 
between  the  lens  capsule  and  that  of  the  inner  ear.  The  nasal 
sacs,  again,  are  similar  capsules  that  do  not  lose  their  connec- 
tion with  the  exterior,  and  it  must  be  remembered  that  in  the 
endolymphatic  duct  of  the  selachians  we  see  the  same  retention 


THE    SENSE-ORGANS  503 

of  the  original  connection.  We  seem  here  almost  able  to  re- 
produce an  important  bit  of  lost  history,  but  the  proofs  are  not 
forthcoming  and  may  always  be  wanting,  since  the  early  phy- 
logenetic  stages  were  probably  passed  in  those  lost  forms  be- 
tween Amphioxus  and  the  selachians,  and  concern  soft  parts, 
no  trace  of  which  is  likely  to  be  found  in  fossil  remains. 

The  absolute  size  of  the  eyeball  is  very  variable.  In  gen- 
eral it  is  somewhat  in  proportion  to  the  size  of  the  body,  yet 
the  eyeballs  of  the  elephant  or  the  whale,  although  large  in 
both  cases,  are  not  proportionate  to  their  enormous  bulk  when 
compared  with  those  of  Man,  for  instance.  Again  there  is  a 
certain  proportion  between  the  size  of  the  eyeball  and  the 
sharpness  of  vision,  as,  for  example,  the  enormous  eyes  of 
birds ;  but  here,  again,  must  be  mentioned  the  small  but  exceed- 
ingly acute  eyes  of  rodents  where  the  decrease  of  size  seems  to 
be  due  to  the  excessive  development  of  the  masseter  muscles, 
and  appears  to  have  no  direct  influence  upon  the  vision.  The 
eyes  are  apt  to  be  large  in  animals  with  nocturnal  vision,  like 
the  lemurs,  and  it  is  possible  that  the  relatively  large  eyes  of 
Man,  which  have  encroached  upon  the  nasal  cavities,  and  thus 
reduced  the  power  of  smelling,  may  be  the  result  of  a  nocturnal 
habit  in  some  not  very  remote  ancestors. 

Of  the  organs  external  to  the  eyeball  which  are  accessory  to 
the  sense  of  sight  the  muscles  have  been  treated  in  a  preceding 
chapter  (Chap.  VI).  There  thus  remain  for  consideration 
only  the  eyelids  and  the  glands,  two  sets  of  structures  closely 
associated  with  one  another.  They  are  both  employed  in  pre- 
venting the  surface  of  the  eyeball  from  becoming  dry  upon 
exposure  to  the  air,  and  belong  to  that  series  of  changes  necessi- 
tated by  the  change  of  environment  from  water  to  air.  They 
are  consequently  found  only  in  the  higher  vertebrates,  and  are 
absent  in  fishes,  and  but  poorly  developed  in  aquatic  urodeles. 

In  fishes  the  integument  fits  smoothly  over  the  region  sur- 
rounding the  eyeball,  and  is  continuous  over  the  latter  as  a 
thin  skin,  usually  transparent,  but  occasionally  ornamented  in 
places  with  pigmented  areas,  which  continue  the  color  scheme 
of  the  rest  of  the  skin.  The  eyelids,  which  appear  first  in 


5o4  HISTORY   OF   THE    HUMAN    BODY 

amphibians,  form  as  dorsal  and  ventral  folds  of  the  integument, 
which  may  become  stiffened,  either  by  connective  tissue  or  by 
cartilage,  as  in  mammals  (tar sal  cartilages).  That  portion  of 
integument  which  forms  the  inner  face  of  the  folds  and  is  con- 
tinued over  the  front  of  the  eyeball  is  very  thin  and  sensitive, 
and  forms  the  conjunctiva.  A  nictitating  membrane  is  formed 
in  some  vertebrates  by  an  inner  fold  of  this  last ;  it  attains  in 
birds  and  in  some  mammals  the  dignity  of  a  third  eyelid;  in 
Man  it  is  represented  by  the  plica  semilunaris,  a  delicate  fold 
situated  in  the  inner  corner. 

The  lubricating  fluid,  the  "tears,"  is  furnished  by  twc 
groups  of  glands  which  arise  as  invaginations  of  the  conjunc- 
tiva and  retain  their  connection  with  that  layer,  supplying  the 
pockets  formed  by  the  lids.  These  are  (i)  the  harderiar, 
glands,  which  are  located  about  the  anterior  (inner)  corner  anc 
are  associated  with  the  nictitating  membrane,  and  (2)  the 
lacrimal  glands,  located  near  the  posterior  (outer)  corner 
This  differentiation  is  not  found  in  the  amphibians  where  the 
glands  are  all  alike  and  are  evenly  distributed,  but  appears  ir 
reptiles,  from  which  point  the  two  groups  are  distinct,  both  ir 
location  and  structure.  The  harderian  glands  are  well  de- 
veloped in  reptiles,  birds  and  most  mammals,  but  are  rudi- 
mentary in  the  Anthropoidea.  The  lacrimal  gland  is  asso- 
ciated in  reptiles  and  birds  with  the  lower  eyelid,  beneath  whicr. 
its  ducts  empty,  but  migrates  in  mammals  to  a  more  dorsad 
position  and  thus  becomes  almost  exclusively  associated  witl 
the  upper  lid.  In  some  mammals  a  few  ducts  occur  in  the 
lower  fold;  indications  of  its  former  location.  The  lacrima. 
fluid,  supplied  by  both  of  these  glands,  is  continually  being  se 
creted  and  is  as  constantly  spread  in  an  even  layer  over  the 
outer  surface  of  the  eyeball  by  the  movement  of  the  lids.  Th< 
excess  is  conveyed  to  the  nasal  cavities  through  the  nasolacrima 
duct,  which  appears  in  amphibian  larvae  as  an  integumenta 
groove  extending  from  eye  to  nostril.  This  eventually  close; 
up,  sinks  into  the  interior,  and  gains  its  independence  from  the 
integument,  thus  forming  an  internal  canal  connecting  the  con- 
junctival  sac  with  the  anterior  end  of  the  nasal  cavity. 


THE    SENSE-ORGANS  505 

Aside  from  these  conjunctival  glands  there  appear  in  mam- 
mals certain  glands  associated  with  the  eyelashes.  These  are 

1 i )  the  tarsal  [meibomian],  which  are  modified  sebaceous,  and 

(2)  the  ciliary,  modified  perspiratory  glands.      These  open 
along  the  edges  of  the  lids  and  produce  narrow  lines  of  oil 
which  repel  the  lacrimal  fluid  and  assist  in  retaining  it  within 
the  peripheral  folds.     As  the  eyelashes  are  modified  hairs,  the 
tarsal  glands  may  be  looked  upon  as  the  associated  sebaceous 
glands,  considerably  hypertrophied,  and  changed  somewhat  in 
their  relation  to  the  hairs. 

This  entire  lacrimal  apparatus,  including  the  glands  and  the 
nasolacrimal  duct,  becomes  much  reduced  in  such  aquatic  mam- 
mals as  the  hippopotamus,  seal  and  otter,  and  in  the  pelagic 
whales  and  porpoises  is  entirely  rudimentary.  In  snakes  there 
occurs  a  singular  adaptation,  which  protects  their  eyes  from 
the  danger  of  the  thick  grasses  and  twigs  by  fusing  the  two 
eyelids  together  over  the  eyeball  and  then  rendering  them  ab- 
solutely transparent.  There  is  thus  formed  a  plate,  in  shape 
like  a  watch  glass,  and  serving  as  a  second  cornea.  This  is 
shed  with  each  successive  skin  and  forms  a  conspicuous  feature 
of  the  exuviae,  or  "  snake-skins,"  objects  commonly  met  with 
in  fields  frequented  by  snakes.  A  lacrimal  apparatus  is 
wholly  wanting. 

As  special  protective  organs  to  the  eye  may  be  mentioned  the 
long  superciliary  bristles,  which  in  cats  and  a  few  mammals 
project  over  the  eye  and  when  touched  cause  the  automatic 
closing  of  the  lids ;  also  the  eyebrows  of  the  higher  anthropoids, 
especially  Man,  the  hairs  of  which  point  outwards  and  curve 
downwards  at  the  outer  end  to  receive  the  perspiration  of  the 
forehead  and  convey  it  away  from  the  eyes. 


CHAPTER   XII 
THE  ANCESTRY  OF  THE  VERTEBRATES 

"  Ainsi  la  plus  ancienne  couche  fossilifere  connue  nous 
montre  des  representants  de  presque  toutes  les  classes 
d'Invertebres.  Cela  demontre  1'existence  d'une  longue 
periode  anterieure  a  celle  sur  laquelle  la  Paleontologie 
peut  nous  fournir  des  renseignements  et  dans  laquelle 
ont  pris  naissance  presque  tous  les  types  actuels. 
Parmi  ces  etres,  dont  les  formes  resteront  tou jours  tin 
mystere,  devaient  se  trouver  les  ancetres  sans  squelette 
des  Vertebres  actuels." 

DELAGE  ET  HEROUARD,  Les  Procordes,  1898,  p.  357, 

PREVIOUS  to  the  establishment  of  the  modern  theory  of  evo- 
lution, which  removed  each  animal  and  plant  from  an  isolated 
position  unrelated  to  the  rest,  and  assigned  to  it  a  place  in  a 
connected  chain  of  organic  beings,  morphological  speculation 
was  limited  to  ingenious  comparisons  with  little  or  no  logical 
basis,  conjured  up  to  explain  real  or  fancied  resemblances. 
Thus  Lorenz  Oken,  having  conceived  the  idea  that  the  head 
must  possess  parts  corresponding  to  those  of  the  trunk,  consid- 
ered the  nasal  cavity,  the  cephalic  thorax ;  the  mouth  cavity,  the 
cephalic  abdomen ;  and  the  palate,  the  cephalic  diaphragm ;  to 
him  the  halves  of  the  upper  and  lower  jaws  represented  re- 
spectively the  anterior  and  posterior  limbs,  in  which  the  teeth 
were  the  digits.  Thus  Geoffrey  St.  Hilaire  compared  insects 
with  vertebrates,  making  the  exoskeletal  rings  the  equivalent  of 
the  vertebrae,  and  the  jointed  legs  that  of  the  ribs.  The  rela- 
tion of  nerve  cord,  intestine,  and  main  blood-vessels  was  made 
the  same  by  placing  the  insect  upon  its  back. 

Others,  like  Goethe  and  Cuvier,  sought  to  base  the  compari- 
son between  different  forms  upon  the  assumption  of  an  arche- 
type (Goethe's  "  Urbild  "),  of  which  a  certain  related  group  of 
animals  might  be  considered  as  so  many  various  modifications. 
//  such  an  archetype  had  been  considered  to  have  or  to  have  had 

506 


THE  ANCESTRY  OF  THE  VERTEBRATES   507 

a  real  existence,  it  would  have  been  the  ancestor  of  the  group  in 
the  modern  sense,  but  there  is  little  to  be  found  in  the  writings 
of  these  early  morphologists  to  suggest  such  a  relationship, 
and  the  archetype  seems  to  have  been  considered  a  mere  ab- 
straction, a  working  hypothesis  in  definite  architectural  form, 
employed  for  the  purpose  of  facilitating  comparison.  There  is 
often  indeed  the  idea  that  the  archetype,  non-existent  in  its  per- 
fection, forms  a  divinely  constructed  plan  upon  which  the 
Creator  has  modeled  each  member  of  a  group  of  organisms,  and 
that  Man  is  able  to  grasp  and  understand  this  plan  through  his 
spiritual  insight,  a  faculty  akin  to  that  of  the  Deity  himself. 
Says  Goethe,  "  Sollte  es  denn  eben  unmoglich  sein,  da  wir 
einmal  anerkennen,  dass  die  schaffende  Gewalt  nach  einem 
allgemeinen  Schema  die  vollkommeneren  organischen  Naturen 
erzeugt  und  entwickelt,  dieses  Urbild,  wo  nicht  den  Sinnen, 
doch  dem  Geiste  darzustellen,  nach  ihm  -als  nach  einer  Norm 
unsere  Beschreibungen  auszuarbeiten  und,  indem  solche  von 
der  Gestalt  der  verschiedenen  Thiere  abgezogen  ware,  die 
verschiedensten  Gestalten  wieder  auf  sie  zuruckzufuhren  ?  "* 

This  employment  of  an  hypothetical  archetype  for  the  com- 
parison of  organisms  reached  its  culmination  in  the  marvelous 
structure  reared  by  the  English  anatomist,  Sir  Richard  Owen, 
who  first  established  his  great  fundamental  conception  of  a 
typical  vertebra,  and  then  described  in  terms  of  this  all  the 
skeletal  parts  of  every  known  vertebrate,  including  here  not  the 
vertebral  column  alone  but  the  skull  and  appendicular  skeleton 
as  well. 

His  diagram  of  a  typical  vertebra,  reproduced  here  (Fig. 
137),  shows  that  he,  too,  as  well  as  so  many  others,  conceived 
of  the  typical  or  primordial  form  as  a  symmetrical  and  perfect 

* "  But  should  it  then  be  impossible,  when  once  we  recognize  that  the 
Creative  Power  has  produced  and  developed  the  more  completely  organ- 
ized natures  after  a  general  plan,  for  us  to  represent  this  Archetype,  if 
not  to  the  senses,  at  least  to  the  mind,  to  elaborate  our  descriptions  in  ac- 
cordance with  it  as  with  a  norm,  and,  since  such  archetypes  were  taken 
from  the  forms  of  different  animals,  to  refer  the  most  varied  forms  back 
to  it  again?"  Johann  Wolfgang  Goethe,  "  Uber  einen  aufzustellenden 
Typus  zu  Erleichterung  der  vergleichenden  Anatomic,"  1796. 


508 


HISTORY    OF    THE    HUMAN    BODY 


one ;  the  Golden  Age  idea  appearing  in  Anatomy,  but  here,  as 
everywhere  else,  the  reverse  of  actual  history.  Yet  this  dia- 
gram, although  erroneous  as  an  explanation  of  early  con- 
ditions, represents  in  a  clear  manner  the  parts  that  appear  in 
actual  cases,  especially  in  the  higher  forms,  and  as  such  has 
been  taken  as  the  foundation  of  our  modern  nomenclature. 
This  typical  vertebra  consists  of  a  cylindrical  centrum,  fur- 
nished with  a  neural  arch,  a  hcemal  arch  and  several  lateral  ele- 


ns 


FIG.  137.     Owen's  original  diagram  of  a  typical  vertebra,  to  illustrate 
his  theory  of  the  archetype.     [After  OWEN.] 

ns,   neural    spine;   s,   zygapophysis ;    np,   neurapophysis ;    d,   diapophysis;    pi,   pleura- 
pophysis;  p,  parapophysis;  hp,   haemapophysis;   hs,   haemal  spine;    h,   haemal  canal. 

ments.  The  neural  arch  consists  of  a  pair  of  neurapophyses 
and  a  neural  spine,  and  bears  a  pair  of  articular  processes,  the 
zygapophyses;  and  in  like  manner  the  haemal  arch  is  composed 
of  a  pair  of  hamapophyses,  a  hcemal  spine  and  a  second  pair  of 
zygapophyses.  Of  the  lateral  pieces  the  central  ones  are  the 
pleurapophyses  or  rib  elements,  sometimes  forming  free  ribs, 
and  dorsal  and  ventral  to  these  lie  respectively  the  diapophyses 
and  parapophyses,  more  occasional  elements. 


THE  ANCESTRY  OP  THE  VERTEBRATES  509 

From  this  typical  vertebra  Owen  was  able  to  explain  the 
skeletal  elements  in  each  segment  of  the  body  in  every  verte- 
brate, and  was  thus  able  to  construct  the  skeleton  of  an  Arche- 


FIG.  138.     Owen's  interpretation  of  mammalian  skulls.     [After  OWEN.] 

(A)     Generalized     mammal.       (B)     Man.       For     explanation     see     accompanying 
table     given    in    text. 

type,  which  consisted  of  a  series  of  such  vertebrae  gradually 
tapering  and  losing  their  characteristic  features  in  the  caudal 


5io  HISTORY   OF   THE    HUMAN    BODY 

region  and  somewhat  modified  also  at  the  anterior  end,  through 
the  development  of  the  brain  and  the  introduction  of  the  sense- 
organs. 

The  skull  was  formed  by  four  of  these  typical  vertebrae, 
called  nasal,  frontal,  parietal  and  occipital.  The  centra  are  rep- 
presented  by  vomer,  presphenoid,  basisphenoid,  and  basi-occipi- 
tal,  the  neurapophyses  by  prefrontals,  orbitosphenoids,  alisphe- 
fioids  and  exoccipitals,  and  the  neural  spines,  composed  mainly 
of  paired  pieces,  by  the  nasals,  the  f  rentals,  the  parietals  and  the 
supraoccipital.  The  postfrontal  was  the  diapophysis  of  the 
frontal  vertebra,  and  the  mastoid  that  of  the  parietal.  Pleura- 
pophyses  were  represented  by  the  palatine,  which  belonged  to 
the  nasal  vertebra,  the  tympanic,  which  belonged  to  the  fron- 
tal, the  stylohyals,  parts  of  the  parietal  vertebra,  and  the 
suprascapula  and  scapula,  which  were  reckoned  with  the  oc- 
cipital. The  hcemopophyses  were  respectively  represented  by 
the  maxillaries,  the  articularia,  the  ceratohyals  and  the  cora- 
coids,  and  the  haemal  spines  by  the  premaxillaries,  the  dentaries, 
the  basihyals  and  the  episternum.  Other  parts,  such  as  the 
squamosal,  the  thyreohyal,  and  the  free  limb,  formed  ele- 
ments called  appendages. 

Never  was  there  a  more  stupendous  result  of  the  labor 
of  a  single  human  life  than  this  great  work  of  Owen,  and  yet 
of  the  entire  structure  reared  by  his  incessant  toil  all  that  re- 
mains is  the  large  amount  of  accurate  description  and  a  great 
enrichment  of  osteological  nomenclature.  It  was  a  house  built 
upon  the  sand,  and  Owen's  "  typical  vertebra  "  may  be  placed 
alongside  of  Goethe's  "  Urbild  "  as  the  noble  attempt  to  picture 
the  great  truths  which  they  felt  in  spirit  and  saw  but  dimly. 

The  formulation  by  Charles  Darwin  in  1859  of  the  doctrine 
of  animal  descent,  with  the  implied  conception  of  actual  blood 
relationship  between  the  different  groups,  introduced  into 

The  theory  may  be  further  elucidated  by  the  help  of  the  following  table 
and  by  the  accompanying  diagrams  (Fig.  138,  A  and  B).  The  small  letters 
added  to  the  names  of  the  separate  elements  in  the  table  correspond  to 
those  used  in  the  diagram,  so  that  the  former  may  be  used  to  explain  the 
latter. 


THE  ANCESTRY  OF  THE  VERTEBRATES   511 


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512  HISTORY    OF    THE    HUMAN    BODY 

anatomical  speculation  that  necessary  principle  which  had  been 
lacking  in  the  philosophy  of  pre-Darwinian  anatomists,  and 
from  that  time  on  actual  ancestors  took  the  place  of  theoretical 
archetypes.  It  became  then  of  fundamental  importance  to 
establish  the  true  interrelationships  of  the  various  animal 
groups,  that  the  structure  of  a  given  form  might  be  explained 
in  terms  of  the  ancestral  structure  of  which  it  might  be  consid- 
ered a  modification. 

Naturally  the  intensest  interest  centered  about  the  establish- 
ment of  the  group  from  which  the  vertebrates  were  derived, 
and  here  for  a  long  time  the  most  of  the  speculation  followed 
the  lines  laid  down  by  St.  Hilaire  with  his  reversed  insect. 
Certainly  one  of  the  most  characteristic  features  of  a  verte- 
brate, and  one  of  the  earliest  to  appear  in  the  embryo,  is  the 
division  of  the  body  into  somites,  and  the  search  for  a  bilateral 
segmented  ancestor  must  inevitably  lead  back  to  the  articulates, 
which  alone  of  the  invertebrates  emphasize  this  characteristic  to 
an  equal  degree.  It  is  true  that  in  the  two  groups  the  arrange- 
ment of  the  internal  organs  is  in  the  main  reversed,  for  in  the 
one  the  central  nervous  system  is  ventral  and  the  main  blood- 
vessel dorsal,  and  in  the  other  the  former  is  dorsal  and  the  lat- 
ter ventral ;  but  the  device  employed  by  St.  Hilaire  to  explain 
this  is  by  no  means  an  absurd  one,  since  what  is  called  dorsal 
or  ventral  in  a  given  animal  is  merely  its  constant  physiological 
relation  to  the  surface  of  the  earth,  and  in  several  cases,  like  the 
flounder  and  the  squid,  is  known  to  be  quite  at  variance  with 
the  condition  usual  in  related  forms. 

Thus  by  postulating  the  occurrence  of  a  change  quite  in  ac- 
cord with  several  known  instances  the  differences  in  the  rela- 
tionships of  the  different  systems  in  the  simplest  members  of 
both  groups  (annelids  and  selachians)  may  be  brought  into 
almost  complete  harmony  (Fig.  139,  a  and  c).  Even  the  noto- 
chord,  perhaps  the  greatest  problem  of  vertebrate  structure, 
may  be  compared  to  the  "  Faserstrang,"  a  bundle  of  fibers  run- 
ning along  the  nerve  chain  and  serving  as  a  support.  This 
and  the  notochord  lie  in  a  precisely  similar  position  in  relation 
to  the  other  organs,  and  in  both  cases  they  are  enclosed  with 


THE  ANCESTRY  OF  THE  VERTEBRATES   513 

the  nerve  cord  in  a  common  sheath  of  connective  tissue.  In  the 
blood  system  there  are  equal  points  of  resemblance,  for  in  each 
case  there  are  two  median  longitudinal  vessels,  one  on  either 
side  of  the  intestine.  In  the  vertebrate  the  dorsal  one  is  the 
aorta,  which  sends  the  blood  in  a  posterior  direction,  while  the 
ventral  one,  with  a  current  in  the  reverse  direction,  is  repre- 
sented by  the  embryonic  subintestinal  vein  posteriorly  and  by 
the  heart  anteriorly.  In  an  unreversed  annelid  it  is  true  that  the 
dorsal  blood-vessel  sends  its  blood  from  tail  to  head,  while  in 
l  he  ventral  one  the  blood  flows  from  head  to  tail,  but  by  re- 
versing the  animal  the  correspondence  in  the  direction  of  the 
current  becomes  complete.  The  vertebrate  aorta  is  then  repre- 
sented by  the  original  ventral  (now  dorsal)  vessel  in  which  the 
current  flows  backwards,  and  the  subintestinal  vein  and  heart 
are  represented  by  the  original  dorsal  (now  ventral)  vessel,  in 
both  cases  with  the  current  directed  forwards. 

The  most  convincing  of  the  many  correspondences,  however, 
lies  in  the  nephridial  system  of  annelids  and  selachians,  which 
in  both  cases  consist  of  segmentally  arranged  pairs  of  tubes 
that  open  into  the  ccelom  at  their  inner  ends  by  ciliated  nephro- 
stomes.  There  are  also  close  correspondences  in  the  rela- 
tion of  these  tubules  to  the  germ  glands  and  to  the  ccelom. 
The  more  primitive  type  may  be  considered  to  be  that  found 
in  annelids,  in  which  each  somite  possesses  a  separate  ccelom, 
or  rather  a  pair  of  cceloms,  since  those  of  adjacent  somites  are 
separated  by  transverse  dissepiments,  and  those  of  the  two  sides 
of  the  same  somite  by  median  sagittal  partitions,  which  form 
dorsal  and  ventral  mesenteries.  Each  of  the  compartments 
thus  formed  is  supplied  with  a  single  nephrostome,  the  tubule 
from  which  pierces  the  posterior  dissepiment  and  enters  the 
next  posterior  ccelomic  pocket,  where  it  exhibits  a  convoluted 
portion  and  a  vesicular  enlargement,  and  eventually  opens  di- 
rectly into  the  exterior  by  a  lateral  opening.  (Fig.  139,  a  and 
b.)  The  germ  glands  develop  on  the  anterior  walls  of  the 
ccelomic  cavities,  and  the  germ  cells  become  free  and  float 
about  in  the  ccelomic  fluid  until  they  are  taken  up  by  the 
nephrostome  and  find  their  way  to  the  exterior,  through  the 


514  HISTORY    OF   THE    HUMAN    BODY 


sh 


uph— 


eh 


FIG.   139     Figures  illustrating  the  Annelid  theory.     [After  SEMPER.] 
(a)      Cross      section     of     Annelid      (reversed).      (b)      Longitudinal     section      of 


THE  ANCESTRY  OF  THE  VERTEBRATES   515 

nephridium,  which  thus  serves  as  ductus  deferens  or  oviduct. 
Typically,  as  in  the  diagram,  a  germ  gland  belongs  with  each 
lateral  coelomic  cavity,  but  in  actual  cases  they  develop  in  only 
a  fewy  segments,  and  the  associated  nephridia  become  espe- 
cially modified  for  the  conveyance  of  the  germ  cells  from  the 
body. 

If,  now,  the  primitive  vertebrate  nephridia,  germ-glands  and 
ccelom,  as  described  in  Chapter  IX  above,  be  compared  with 
the  annelid  condition,  the  similarities  are  found  to  be  remark- 
able. Here  also  the  nephridia  are  at  first  strictly  segmental, 
although  they  no  longer  open  to  the  outside  independently,  but 
through  the  medium  of  a  common  Wolffian  duct.  Since,  how- 
ever, this  develops  in  part  from  the  ectoderm,  it  may  have  be- 
gun as  a  simple  external  trough-like  depression  which  ran 
along  the  sides  of  the  animal  and  connected  the  several  indi- 
vidual openings  for  the  better  disposal  of  their  secretion.  The 
segmental  subdivisions  of  the  coelom  are  no  longer  continued 
in  the  higher  vertebrates  but  the  mesodermic  diverticula,  which 
appear  clearly  in  Amphioxus,  and  in  a  more  imperfect  manner 
in  the  others,  suggest  the  former  presence  of  transverse  dissepi- 
ments, and  both  dorsal  and  ventral  mesenteries  actually  persist 
as  far  back  as  the  posterior  boundary  of  the  liver,  beyond  which 
the  ventral  one  disappears.  Nor  can  it  be  said  that  the  trans- 
verse dissipiments  are  wholly  lacking,  since,  although  they  no 
longer  divide  the  coelomic  cavity,  they  are  still  represented  in 
the  body  wall  by  the  intermuscular  septa  (myocommata)  with 
which  the  nephridia  sustain  in  the  embryo  similar  relationships 
as  in  annelids  (Fig.  139,  d) .  In  both  cases  the  germ  glands 
arise  as  localized  portions  of  the  ccelom  (peritoneum),  and  the 
presence  of  a  single  pair  in  the  true  vertebrates  may  be  corre- 
lated with  the  confluence  of  the  several  coelomic  cavities  into 
a  single  one.  The  larger  number  of  gonads  in  Amphioxus 
indicates  the  former  presence  of  a  much  larger  number  of 
coelomic  cavities.  That  in  vertebrates  as  in  annelids  the 


Annelid.      (c)    Cross    section    of    Selachian.       (d)    Longitudinal    section    of    Selachian 
in    region    of    kidney. 

n,  nerve  cord;  nc,  notochord;  g,  "  faserstrang "  of  Annelid;  sh,  sheath  sur- 
rounding nerve  cord  and  notochord;  d  and  r,  muscle  masses;  en,  intestine;  nph, 
nephridium;  a  and  b,  longitudinal  blood  vessels. 


516  HISTORY   OF   THE    HUMAN    BODY 

nephridial  system  is  made  to  furnish  channels  of  exit  for  the 
germ  cells  has  been  already  shown  (Chapter  IX),  and  the  open- 
ing into  the  oviduct  has  been  homologized  with  a  prone- 
phridial  nephrostome,  while  in  the  male  the  entire  anterior 
portion  of  the  mesonephros  and  its  duct  becomes  utilized  for 
the  passage  of  the  seminal  fluid. 

One  of  the  most  fundamental  characteristics  of  vertebrates  is 
the  presence  of  paired  gill-slits,  extending  in  two  lateral  rows 
along  the  pharyngeal  region  and  forming  direct  communica- 
tions between  the  pharynx  and  the  exterior;  these  may  be 
readily  derived  from  nephridia  by  supposing,  first,  that  the 
inner  ends  of  these  tubes  become  secondarily  connected  with 
the  pharyngeal  lumen,  and  secondly,  that  the  tubules  become 
reduced  in  length  until  ectoderm  and  endoderm  come  in  con- 
tact. Only  in  some  such  way  can  one  explain  the  embryonic 
development  of  gill-slits  from  a  series  of  ectodermic  inpushings 
that  meet  a  similar  series  of  endodermic  outpushings,  a  mechan- 
ical process  that  necessitates  some  reason  back  of  that  which  is 
apparent  in  order  to  account  for  the  accuracy  with  which  these 
several  folds  meet  one  another.  In  the  embryo  of  the  cyclo- 
stome  Myxine,  precisely  the  form  where  we  would  look  for  the 
retention  of  the  earliest  phases,  there  still  appears  at  first  a 
fairly  long  canal  between  each  ectodermic  inpushing  and  its 
endodermic  associate,  perhaps  a  remnant  of  the  nephridial  tube. 
It  may  also  be  more  than  a  coincidence  that,  when  genuine 
nephridia  of  the  pronephrotic  system  appear  immediately 
posterior  to  the  gill  region,  none  arise  in  the  somites  that  de- 
velop the  gill-slits. 

Important  changes  seem  to  have  taken  place  in  both  outlets 
of  the  alimentary  canal,  and  indications  show  that  vertebrates 
have  acquired  both  a  new  mouth  and  a  new  anus,  although  they 
still  retain  in  the  embryo  many  traces  of  the  older  organs. 
That  the  mouth  of  the  vertebrates  is  not  the  primitive  one  is 
shown  by  a  variety  of  indications,  one  of  the  strongest  being  its 
exceptionally  late  appearance  in  embryonic  life.  A  mouth  is 
one  of  the  most  essential  of  organs,  and  in  other  animals,  cor- 
responding to  its  important  function,  is  one  of  the  first  to 


THE  ANCESTRY  OF  THE  VERTEBRATES  517 

appear.  In  vertebrates,  however,  the  reverse  is  true;  the 
nervous  system  is  laid  down,  the  brain  is  differentiated,  the 
notochord  is  formed,  even  the  special  sense-organs  ap- 
pear, and  still  the  alimentary  canal  remains  a  sealed  cavity, 
without  communication  with  the  exterior.  At  last  a  mouth 
appears,  placed  very  far  ventrally,  in  line  with  the  gill-slits, 
and  in  certain  fishes  appears  first  as  two  lateral  openings  which 
eventually  become  confluent.  All  this  seems  a  complete  cor- 
roboration  of  the  fact  arrived  at  independently  through  adult 
anatomy  that  the  vertebrate  mouth  has  resulted  from  the  con- 
fluence of  a  pair  of  gill-slits  anterior  to  those  now  functioning, 
and  still  equipped  with  gill-arches  \vhich  serve  as  jaws.  There 
comes,  then,  the  inevitable  conclusion  that  previous  to  this  con- 
version, and  while  the  mandibular  slits  were  still  functioning  as 
gill-slits,  the  ancestral  forms  must  have  had  another  mouth, 
traces  of  which  are  to  be  looked  for  in  the  earlier  embryo. 
Such  a  primary  mouth  is  actually  found  indicated  in  precisely 
the  place  where  it  would  be  looked  for  in  the  annelid,  taking  the 
reversal  of  the  body  into  account,  and  this  indication  appears 
in  the  widely  open  "  fourth  ventricle  "  of  the  nerve  cord. 

In  annelids,  as  in  all  Articulata,  the  mouth  is  upon  the  ven- 
tral side,  and,  since  the  alimentary  canal  is  dorsal  to  the  nerv- 
ous system,  this  position  is  reached  by  means  of  an  oesophagus, 
which  turns  downwards  almost  at  right  angles  to  the  remainder 
of  the  canal,  and  runs  between  the  two  nerve  connectives  that 
connect  the  first  and  second  pairs  of  ganglia.  The  first  gangli- 
onic  pair  thus  becomes  the  supra-cesophageal,  the  second, 
infra-Ksophageal,  and  the  connections  between  them  form  a 
circum-ocsophageal  ring  through  which  the  oesophagus  passes. 
These  relationships  will  be  clearly  seen  by  reversing  the  accom- 
panying figure  (Fig.  140),  which  will  thus  give  the  conditions 
as  seen  in  annelids.  In  all  true  vertebrates  the  actual  external 
opening  of  this  early  mouth  has  disappeared,  but  it  may  be 
identical  with  the  neuropore  in  the  embryo  of  Amphioxus, 
which  forms  in  this  place  a  direct  communication  between  the 
lumen  of  the  neural  tube  and  the  exterior  and  is  otherwise  un- 
accounted for.  Aside  from  the  indications  of  the  early  mouth 


5i8  HISTORY   OF   THE    HUMAN    BODY 

and  its  oesophagus,  furnished  by  fourth  ventricle  and  neuro- 
pore,  there  is  also  the  hypophysis,  or  rather,  that  portion  of  it 
that  is  pushed  up  from  the  alimentary  canal,  for  which  there  is 
yet  no  satisfactory  explanation.  Its  origin  from  the  alimentary 
canal,  its  constant  appearance  in  all  vertebrates,  its  relationship 
to  the  nervous  system  and  its  position,  all  suggest  that  it  also 
is  a  remnant  of  an  early  oesophagus. 

The  formation  of  a  new,  ventrally  placed  anus  is  due  to  a 
procedure  similar  to  that  which  forms  the  new  mouth,  although 
there  is  here  no  suggestion  of  a  gilt-slit.  The  vertebrate  anus 
arises  as  a  mid-ventral  inpushing  of  the  ectoderm  at  some  dis- 
tance from  the  end  of  the  tail,  and  thus  reaches  the  primary 
intestine  along  its  course,  leaving  beyond  it  a  piece  of  consid- 
erable length,  the  post-anal  gut,  which  soon  atrophies.  This 


HI 


FIG.  140.     Reversible  diagram  illustrating  the  Annelid  theory. 

Reversible  designations,  applying  to  both  forms:  S,  brain;  X,  nerve  cord; 
H,  alimentary  canal.  Designations  applying  to  Annelid  only:  m,  mouth;  a,  anus. 
Designations  applying  to  Vertebrate  only;  st,  stomatodaeum;  pr,  proctodaeum; 
nt,  notochord. 

phenomenon,  inexplicable  by  other  means,  is  easily  explained 
by  the  postulate  of  an  annelid  ancestor,  for  in  these  animals  the 
anus  is  at  the  posterior  extremity  of  the  body,  and  the  forma- 
tion of  a  new  anus  in  the  vertebrate  position  would  actually 
leave  just  such  a  piece  as  the  one  in  question. 

The  gills  of  aquatic  vertebrates  receive  also  an  adequate  ex- 
planation through  the  annelid  hypothesis.  The  annelid  gills 
are  external  duplicatures  of  the  integument,  and  occur  upon 
the  sides  of  every  somite,  attached  to  the  parapodia,  or  short 
locomotor  organs.  In  simple  forms  they  are  plates,  but  when 
specialized  they  become  fringed  or  dendritic  and  somewhat  re- 
semble the  external  gill-bushes  of  amphibians.  Although  pri- 
marily distributed  along  the  entire  body,  in  certain  specialized 


THE  ANCESTRY  OF  THE  VERTEBRATES   519 

forms  they  are  confined  to  the  anterior  end.  Gills  of  this  sort 
are  well  adapted  to  slow-moving  or  crawling  forms,  but  when 
there  is  a  necessity  for  the  development  of  rapid  motion,  as  is 
indicated  for  the  direct  ancestor  of  the  rapidly  moving  fishes, 
such  gills,  especially  if  long  and  fringed,  would  tend  to  retard 
the  motion.  It  would  thus  be  natural  to  consider  that  they 
might  wander  within  the  openings  of  the  nephridia,  which  in 
annelids  lie  close  to  these  external  gills,  and  this  relationship 
gives,  in  its  turn,  the  motive  for  the  secondary  connection  of 
such  nephridia  with  the  alimentary  canal,  in  order  to  supply 
the  gills  with  a  current  of  water. 

The  increase  in  the  size  of  the  gills  would  tend  to  develop 
some  firmer  tissue  at  their  base  to  support  them,  and  in  this  way 
there  may  have  been  developed  a  series  of  cartilaginous  arches, 
which,  together  with  the  gills  themselves,  may  have  been  at 
first  and  for  a  long  time  coextensive  with  the  body  itself,  or 
have  extended  at  least  as  far  as  the  anus.  When  at  a  later 
period  the  gills  became  restricted  to  a  few  anterior  pairs  while 
the  rest  atrophied,  the  arches  accompanying  the  former  would 
be  the  persistent  gill-arches,  and  form  the  visceral  skeleton 
of  vertebrates,  while  the  remainder,  freed  from  their  gills, 
and  repeating  themselves  metamerically,  would  become  the  ribs. 
It  is  even  permissible  to  conceive  of  the  limb  skeletons  as 
further  derivatives  of  the  metameric  system  of  gill-arches ;  per- 
haps also  the  original  elements  of  the  primordial  skull,  the 
trabeculse  and  parachordals,  may  be  traced  to  the  same  source, 
thus  accounting  for  all  parts  of  the  skeleton  save  the  dermal 
bones,  which  are  integumental,  and  the  notochord,  which  has 
already  been  accounted  for. 

Convincing  as  these  comparisons  seem  when  taken  by  them- 
selves, the  influence  of  later  investigation  has  tended  rather 
away  from  the  annelid  hypothesis,  and  at  present,  although 
there  are  many  investigators  who  seek  the  ancestor  of  verte- 
brates in  some  worm-like  form,  there  are  few  who  wish  to  defi- 
nitely assert  that  this  ancestor  was  an  annelid. 

The  annelid  theory  rests  largely  upon  the  definite  body  seg- 
mentation of  both  these  animals  and  vertebrates,  yet  segmenta- 


520  HISTORY   OF   THE   HUMAN    BODY 

tion  is  not  in  itself  as  fundamental  a  character  as  would  appear 
at  first,  and  may  be  easily  acquired  by  an  animal  group  in 
any  one  of  several  different  ways.  It  is  likely,  for  example, 
that  such  a  segmentation  as  that  possessed  by  vertebrates  may 
have  been  gained  through  the  muscular  action  of  a  previously 
unsegmented  form,  and  the  fact  that  in  vertebrate  embryos 
the  segmentation  first  appears  in  the  mesoderm,  from  which 
the  muscles  are  derived,  furnishes  a  strong  support  for  this 
view.  The  oldest  of  the  annelids,  on  the  other  hand,  begins 
life  as  an  unsegmented  larva,  upon  which  the  somites  become 
developed  one  after  another  through  a  sort  of  budding,  a 
process  totally  unlike  that  in  which  the  vertebrate  initiates  its 
segmentation. 

A  second  group  of  vermian  forms  from  which  the  vertebrates 
may  have  developed  is  that  of  the  nemerteans,  a  group  of 
mainly  marine  worms,  of  uncertain  affinities,  but  probably 
allied  to  the  platyhelminths  (flat-worms).  Here  the  nervous 
system  is  not  a  ventral  one,  but  consists  of  two  lateral  cords  im- 
bedded in  the  body  wall,  and  often  a  smaller  mid-dorsal  cord, 
the  three  being  bound  together  by  commissural  nerves  which 
run  around  the  animal  (Fig.  141,  A).  A  branching  intesti- 
nal nerve  proceeds  from  one  of  these  and  is  distributed  to  the 
sides  of  the  intestine;  and  from  the  ventraLportion  of  some  of 
the  anterior  commissural  nerves  small  nerve  branches  appear, 
also  distributed  to  the  intestinal  wall.  The  anterior  end  of 
each  lateral  nerve  is  enlarged  into  a  ganglion,  from  which  a 
few  nerves  proceed  anteriorly. 

The  manner  in  which  such  a  nervous  system  may  become 
converted  into  that  of  a  typical  vertebrate  may  be  readily  seen 
by  a  comparison  of  A  and  B  of  Fig.  141,  the  first  of  which 
has  already  been  referred  to.  Of  the  three  longitudinal  nerves 
the  dorsal  one  has  become  the  central  nervous  system,  and  has 
expanded  its  anterior  end  into  a  brain,  while  the  two  lateral 
nerves  have  become  subordinated  to  it,  but  persist  in  lower 
vertebrates  as  the  lateral  nerves  of  the  Vagus  system,  rami 
laterales  X,  with  which  the  long  intestinal  nerve  is  also  asso- 
ciated. The  original  ganglion  of  the  lateral  nerves  breaks  up 


THE  ANCESTRY  OF  -  THE  VERTEBRATES   521 

into  the  various  ganglia  found  in  vertebrates,  in  association 
with  the  cranial  nerves.  As  for  the  commissural  nerves,  they 
become  alternately  sensory  and  motor  in  function  and  associate 
together  in  pairs,  forming  the  metamerically  arranged  spinal 
nerves  of  vertebrates,  the  elements  of  which  are  in  lower  forms 
still  separate  and  issue  from  the  neural  canal  through  separate 
foramina.  Lastly  a  sympathetic  system  is  formed  by  collect- 


B 


FIG.  141.  Nemertean  theory  of  the  origin  of  Vertebrates.  [After 
HUBRECHT.] 

(A)  Typical  diagram  of  Nemertean.  d,  dorsal  nerve  cord;  gl,  ganglion;  It, 
lateral  nerve  cord;  v,  intestinal  nerve;  sb,  small  intestinal  branches. 

(B)  Typical  diagram  of  Vertebrate;  db,  brain;  d,  dorsal  nerve  cord;  s,  sensory,  and 
m,  motor  spinal  nerves;  gl,  sympathetic  ganglia;  -v,  ramus  intestinalis  vagi;  It,  ramus 
lateralis  vagi;  sb,  sympathetic  branches. 

ing  together  the  small  intestinal  branches  that  come  from  the 
ventral  portions  o'f  the  commissural  nerves. 

Concerning  the  other  systems  it  is  only  fair  to  say  that  their 
correspondence  is  by  no  means  as  close  as  is  that  of  the  nervous 
system,  although  a  characteristic  nemertean  structure,  the 
proboscis,  has  been  likened  to  the  hypophysis,  while  its  sheath, 
into  which  it  may  be  retracted,  has  been  cited  as  possibly  fur- 


522  HISTORY   OF    THE    HUMAN    BODY 

nishing  material  for  the  notochord.  Attention  has  also  been 
called  to  the  respiratory  function  of  the  anterior  portion  of  the 
intestinal  canal  in  nemerteans. 

Aside  from  all  hypotheses  which  have  at  their  basis  the  con- 
sideration of  a  worm-like  ancestor  may  be  briefly  mentioned  a 
recent  theory  which  finds  the  vertebrate  ancestor  among  the 
more  primitive  arachnoids,  now  represented  by  such  animals 
as  the  scorpion  and  the  horse-shoe  crab  (Limulus)  and  for- 
merly exhibited  by  the  extinct  group  of  Merostomata.  To  ap- 
preciate this  one  must  at  the  outset  dispose  of  the  cyclostomes 
and 'other  low  forms  like  Amphioxus  as  degenerate  and  with- 
out special  significance,  and  take  as  the  starting  point  of  verte- 
brates such  forms  as  the  ganoids,  or  more  especially  the  placo- 
derms,  which  lived  in  Devonian  times  and  were  contemporaries 
of  certain  aquatic  arachnoids,  allies  of  the  horse-shoe  crab. 

As  the  starting  point  in  this  theory  there  may  be  taken  a 
certain  series  of  resemblances  between  the  brain  and  cranial 
nerves  of  vertebrates  and  the  fused  cephalo-thoracic  gangl  ionic 
mass  found  in  such  arachnoids  as  the  scorpion  and  the  horse- 
shoe crab.  In  these  forms  this  central  mass  is  divisible  into 
three  distinct  portions,  comparable  to  fore-,  middle-  and  hind- 
brains,  with  an  accessory  part  corresponding  to  the  medulla. 
The  number  of  neuromeres,  or  primary  nerve  somites  of  which 
these  parts  are  composed,  i,  e.,  3-1-5  for  the  brain  and  2  to  4 
for  the  medulla,  also  corresponds  closely  with  tfie  conclusion  of 
many  specialists  concerning  the  segmental  values  of  those  parts 
of  the  head  in  vertebrates.  A  similarly  suggestive  resemblance 
exists  in  the  cranial  nerves  and  the  relations  of  the  organs  of 
sense. 

Although  the  anatomy  of  the  soft  parts  of  the  Merostomata 
will  never  be  known,  they  could  not  have  been  very  different 
from  the  condition  found  in  Limulus  and  the  scorpion,  and  it 
may  even  be  supposed  that  they  and  modern  vertebrates  have 
developed  in  distinctly  different  directions  from  these  as  com- 
mon ancestors,  and  that  thus  their  condition  may  have  been  far 
more  like  that  of  the  vertebrates  than  is  that  of  any  of  the 
arachnoids  now  living.  Aside  from  the  nervous  system, 


THE  ANCESTRY  OF  THE  VERTEBRATES   523 

numerous  other  parts  are  more  or  less  comparable.  For  ex- 
ample, the  eyes  of  the  Merostomata  consisted  of  a  pair  of 
widely  divergent  lateral  eyes,  and  a  pair  of  closely  ap- 
proximated median  eyes,  a  peculiarity  seen  in  the  present- 
day  Limnlits;  in  the  vertebrates  there  are  the  same  widely 
divergent  lateral  eyes,  and  the  median  pair  is  well  repre- 
sented by  the  pineal  eye,  in  the  development  of  which  there  are 
many  suggestions  of  its  having  been  double  at  an  earlier 
period.  In  Limulus  the  central  nerve-complex  is  protected  by 


•V 

FIG.  142.     Comparison  of  heart  and  gill  arches  of  (a)  Arachnoid,  and 
<b)  Vertebrate.     [After  PATTEN.] 

an  internal  skeletal  piece,  variously  called  "  sternum "  and 
"  endocranium,"  which  in  general  form  and  still  more  in  its  re- 
lationships to  other  parts  resembles  the  primordial  skull  of 
vertebrates.  The  arterial  system  shows  in  each  case  a  tubular, 
somewhat  contorted  heart,  an  arterial  trunk  proceeding  anteri- 
orly from  this,  and  dividing  into  pairs  of  arterial  arches,  from 
which,  after  sending  a  branch  to  the  head,  the  blood  is  re- 
collected into  two  lateral  channels,  in  the  one  case  a  sinus, 


HISTORY   OF    THE    HUMAN    BODY 

in  the  other  a  definite  vessel.  This  relationship  suggests  a  cer- 
tain homology  between  the  appendages  of  Limulus  and  the  gill- 
arches  of  the  vertebrates,  and  as  a  matter  of  fact  it  has  been 
suggested  that  the  second  pair  of  arachnoid  appendages,  so 
largely  developed  in  the  scorpion,  do  actually  represent  the 
mandibular  arch,  and  that  the  next  pair,  or  perhaps  the  next 
two,  may  represent  the  hyoid.  The  vertebrate  gills  them- 


FIG.   143.     Comparison  between  extinct  Crustacean  and  Vertebrate. 

(A)  Gigantostracan,  Eurypterus  [after  NIEDZKOWSKY].  (B)  Placoderm,  Pterichthys 
[after  NEUMAYER]. 

selves  seem,  however,  more  nearly  comparable  in  structure  with 
the  gill-plates  or  gill-books  of  modern  arachnoids,  and  their 
more  posterior  position  in  those  latter  is  accounted  for  through 
a  backward  migration,  since  the  nerves  supplying  them  are 
precisely  the  ones  which,  for  other  reasons,  have  been  com- 
pared with  Vagus  elements. 

Perhaps  one  of  the  strong  points  in  this  erratic  theory  lies 
in  the  fact  that  the  vertebrates  are  here  derived,  not  from  a 
primitive  segmented  form,  like  an  annelid,  in  which  the  somites 


THE  ANCESTRY  OF  THE  VERTEBRATES  525 

are  much  alike,  but  from  one  in  which  the  differentiation  of 
the  head  somites  and  their  grouping  to  form  complexes  has 
already  progressed  quite  far,  and  apparently  along  the  same 
line.  The  external  resemblance  between  the  heavily  armored 
placoderm  fishes  and  their  contemporaries  among  the  arach- 
noids is  certainly  striking,  and  the  similarity  extends  also  in 
the  head  region  to  the  plates  of  which  the  shell  or  cephalo- 
thorax  is  composed  (Fig.  143). 

Without  subjecting  the  arachnoid  theory  to  further  com- 
ment other  than  to  say  that  it  has  received  very  little  recog- 
nition, we  may  pass  to  that  theory  which  places  especial  weight 
upon  the  notochord,  the  gill-slits  and  the  dorsal  position  of  the 
central  nervous  system,  and  by  means  of  these  has  traced  the 
line  of  vertebrate  ancestry  through  a  series  of  invertebrate' 


"Y 

FIG    144.     Amphioxus. 

o,  oral  hood  with  cirrhi;  x,  mouth;  g,  gonads;  y,  atriopore;  2,   anus;   t,  caudal  fin; 
f,  dorsal  fin. 

lorms,  externally  very  unlike  one  another,  and  each  some- 
what isolated  in  its  systematic  position.  The  first  of  these 
in  descending  series,  and  representing  in  a  way  a  simplified 
vertebrate,  stripped  of  everything  save  the  essentials,  is  the 
now  famous  Amphioxus  (or  Branchiostoma) ,  first  de- 
scribed in  1778  as  a  shell-less  snail,  or  slug  (Limax  lance- 
olatus).  It  is  a  shore  form,  and,  with  a  few  specific  differ- 
ences, occurs  on  almost  all  coasts.  It  is  one  or  two  inches  in 
length,  flattened  laterally,  and  pointed  at  both  ends ;  it  is  thus 
without  a  distinct  head,  but  the  mouth,  which  is  situated  a  little 
ventrally  at  the  anterior  end,  is  equipped  with  a  membranous 
expansion  in  the  form  of  a  hood.  The  adult  burrows  perpen- 
dicularly into  the  sand,  leaving  exposed  only  the  anterior  end, 
and  in  this  temporarily  sessile  condition  it  expands  the  oral 
hood  and  collects  the  debris  that  drifts  past  it,  much  after 


526  HISTORY   OF    THE    HUMAN    BODY 

the  manner  of  other  non-locomotive  forms.  The  larva  is,  how- 
ever, actively  free-swimming,  and  the  adult  often  changes  its 
locality  and  thus  retains  the  power  of  rapid  motion. 

A  striking  external  feature  of  Amphioxus  is  the  regular  seg- 
mentation of  its  muscular  system,  which  shows  through  the 
transparent  skin  and  is  marked  by  lines  formed  by  the  myocom- 
mata,  each  bent  in  the  form  of  a  V,  the  point  directed  for- 
wards. The  reproductive  organs  or  gonads,  also,  are  often 
sufficiently  developed  to  appear  through  the  skin  as  a  succession 
of  square  or  slightly  rounded  masses,  each  corresponding  to  a 
segment,  yet  with  those  of  the  two  sides  placed  alternately, 
as  is  also  the  case  with  the  myomeres  and  the  other  lateral 
parts.  There  is  a  slightly  indicated  median  fin,  supported 
by  minute  fin  rays,  and  extending  along  the  entire  back, 
around  the  tail  and  upon  the  ventral  side  considerably  past  the 
anus,  throwing  this  latter  opening  out  of  the  median  line,  and 
dislocating  it  to  the  left.  Anterior  to  this  and  at  the  termina- 
tion of  the  fin  is  a  large  and  conspicuous  opening,  the  atriopore, 
through  which  the  water  that  is  continually  taken  in  at  the 
mouth  is  as  continually  expelled.  The  chamber  from  which 
the  atripore  leads  is  termed  peribranchial,  and  appears  at  first 
like  an  internal  cavity;  a  little  investigation,  however,  shows 
that  it  is  in  reality  external  and  develops  in  the  larva  from  two 
longitudinal  folds  that  arise  along  the  sides  and  grow  together 
ventrally.  The  region  of  the  body  which  they  enclose  is 
perforated  laterally  by  a  very  large  number  of  obliquely  placed 
gill-slits,  communicating  with  the  pharynx,  so  that  the  water 
taken  in  at  the  mouth  passes  through  these  slits  and  thus  enters 
the  peribranchial  chamber,  from  which  it  is  finally  expelled 
through  the  atriopore.  A  similar  device  is  found  in  frog  and 
toad  tadpoles,  and  the  external  outlet,  here  called  the  "  branchi- 
pore,"  varies  in  position  in  the  different  genera,  but  appears 
quite  high  up  on  the  left  side  in  the  true  frogs  (Rana).  As 
the  peribranchial  chamber  develops  during  larval  life,  the  gill- 
slits  of  young  larvae  open  directly  to  the  outside,  as  in  true 
vertebrates,  and  suggest  that,  as  in  the  frog  tadpole,  this 
chamber  has  been  developed  as  a  special  adaptation  to  the 


THE  ANCESTRY  OF  THE  VERTEBRATES   527 

needs  of  this  particular  animal,  and  is  thus  not  ancestral  in 
character. 

Where  the  two  lateral  folds  that  form  the '  peribranchial 
chamber  come  together  they  unite  in  such  a  way  as  to  leave 
their  original  edges  in  the  form  of  a  pair  of  parallel  metapleural 
folds,  and  it  is  quite  possible  that  either  these,  or  more  probably 
the  original  folds  before  they  extended  far  enough  to  unite 
ventrally,  are  identical  with  those  folds  from  which,  at  some 
period  in  the  long  history  between  Amphioxus  and  the  fishes, 
the  two  pairs  of  limbs  were  originally  derived.  [Cf.  Chap- 
ter V.] 

Of  the  internal  organs  the  most  conspicuous  is  the  noto- 
chord. This  is  an  elastic  skeletal  rod  of  gelatinous  tissue,  sur- 
rounded by  a  firm  connective  tissue  sheath.  It  extends  to  the 
extreme  ends  of  the  animal  and  insures  to  it  a  certain  grade  of 
rigidity  while  allowing  the  body  to  be  extremely  flexible. 
Lying  along  the  dorsal  aspect  of  this  rod,  and  enclosed  in  a 
sheath  which  is  continuous  with  that  of  the  notochord,  is  the 
dorsal  nervous  system,  closely  resembling  that  of  fishes,  but 
with  no  brain  other  than  a  slight  club-shaped  enlargement  at 
the  anterior  end,  the  archencephalon.  An  olfactory  pit  on  the 
left  side,  and  a  median  pigment  spot,  are  its  only  definite  sense- 
organs.  Beneath  the  notochord  lies  the  alimentary  canal, 
which  expands  beyond  the  mouth  into  a  pharynx,  that  extends 
more  than  half  the  length  of  the  body  and  passes  into  a  straight 
intestine  with  no  especial  differentiation  of  parts.  The  pharynx 
is  perforated  by  60-80  pairs  of  narrow  gill-slits,  placed  ob- 
liquely, and  kept  open  by  an  elaborate  system  of  skeletal  rods, 
formed  of  a  material  resembling  chitin,  and  thus  more  like 
an  invertebrate  than  a  vertebrate  structure.  Both  the  elab- 
orateness of  this  skeletal  system  and  the  very  large  number  of 
gill-slits  are  plainly  secondary  modifications,  like  the  peri- 
branchial  chamber,  since  they  are  not  found  in  the  larva,  and 
mark  Amphioxus  as  a  much  modified  form,  probably  that  one 
out  of  a  large  class  which  survived  on  account  of  these  very 
modifications. 

A    characteristic    structure    runs    along   the    floor    of    the 


528  HISTORY   OF   THE    HUMAN    BODY 

pharynx  in  the  form  of  a  ventral  groove.  This  is  the  endo- 
style,  an  organ  that  performs  a  very  important  service  in  the 
collection  and  retention  of  food.  Its  surface  is  covered  with 
cilia,  which  produce  a  backward  directed  current,  and  it  is 
furnished  also  with  gland  cells  that  secrete  a  viscous  fluid. 
The  nutrient  particles  that  enter  the  pharynx  in  the  water 
current  are  engaged  by  the  viscous  fluid,  and  the  combined 
mass  is  conveyed  to  the  intestine  by  the  motion  of  the  cilia. 

The  main  blood-vessels  consist  of  a  sub-intestinal  vein,  ven- 
tral to  the  alimentary  canal,  in  which  the  current  is  directed 
forwards,  and  an  aorta,  situated  between  the  intestine  and  the 
notochord,  in  which  the  blood  flows  posteriorly.  Aside  from 
these  there  is  a  series  of  branchial  vessels,  a  pulsating  heart, 
and  other  vessels,  the  relations  of  which  are  like  that  found  in 
vertebrates,  but  simpler. 

The  general  impression  given  of  the  structure  of  Amphioxus 
is  that  of  a  diagrammatic  vertebrate  modified  by  several  sec- 
ondary adaptations.  Had  it  been  known  to  Goethe,  he  would 
have  almost  denominated  it  the  realization  of  the  primordial 
vertebrate,  an  incarnate  "  Urbild."  In  the  general  arrange- 
ment of  the  essential  organs,  the  dorsal  nervous  system  in 
tubular  form,  the  notochord,  the  dorsal  aorta,  the  intestine 
with  its  pharynx  perforated  by  gill-slits,  it  is  essentially  verte- 
brate; while  in  its  peribranchial  chamber,  and  its  great  multi- 
plication of  gill-slits  it  suggests  the  successful  attempt  of  an 
animal  to  survive  through  the  power  of  acquiring  secondary 
adaptations,  the  only  one  out  of  a  large  group  that  has  come 
down  to  us.  The  segmentally  arranged  muscles  are  essentially 
vertebrate,  and  a  series  of  rather  complex  nephridia  in  the 
gill  region  may  be  homologous  with  a  pronephros.  Even  those 
mysterious  organs  of  true  vertebrates,  the  thymus,  thyreoid, 
and  epithelial  bodies,  the  history  of  which  begins  in  the 
cyclostomes  with  a  series  of  segmental  anlagen,  are  probably 
seen  here  in  an  earlier  stage,  for  it  has  been  suggested  that  the 
endostyle  is  the  homolog  of  the  thyreoid  series,  and  the 
numerous  thymus  anlagen  have  been  rather  hesitatingly  identi- 
fied with  certain  elements  of  the  gill  skeleton. 


THE  ANCESTRY  OF  THE  VERTEBRATES   529 

There  seems  thus  no  doubt  that  in  Amphioxus  we  have  a 
genuine,  although  somewhat  modified,  ancestor  of  the  verte- 
brate group,  the  sole  survivor  of  a  lost  race,  the  more  typical 
members  of  which  were  probably  simpler  and  more  like  the 
true  vertebrates  than  is  their  present-day  representative.  This 
conclusion  serves,  however,  only  to  put  the  question  one  stage 
further  back,  and  we  now  ask  it  in  this  form:  What  is  the 
relation  of  Amphioxus  to  other  invertebrates?  In  order  to  sum- 
marize the  most  fundamental  characteristics  of  Amphioxus, 
those  should  be  selected  which  it  possesses  in  common  with 
the  vertebrates,  since  characters  possessed  by  Amphioxus  alone 
may,  with  great  probability,  be  considered  secondarv  modifi- 
cations, and  thus  unrepresented  in  their  ancestors. 

Thus  reduced,  search  should  be  made  for  an  animal  with  a 
notochord,  a  dorsal  nervous  system,  and  a  pharynx  perforated 
by  gill-slits,  and  possessed  of  a  mid-ventral  endostyle.  Some 
have  included  among  the  essentials  a  pronounced  segmentation, 
that  shows  itself  in  the  muscular  and  nephridial  systems,  but 
a  study  of  segmentation  in  general  leads  to  the  opinion  that 
the  segmentation  of  an  animal,  either  expressed  in  the  body- 
wall,  or  by  the  repetition  of  some  of  the  organs,  is  not  a 
fundamental  character,  but  one  easily  acquired,  and  thus  is 
not  to  be  considered  necessarily  as  an  essential  characteristic 
of  the  group  from  which  Amphioxus  and  its  allies  have  come. 

The  direction  of  the  search  thus  defined,  one  is  led  inevitably 
to  another  isolated  and  problematic  group  of  invertebrates, 
which  have  been  variously  classified  among  molluscs,  worms, 
and  other  comprehensive  and  noncommittal  groups,  which 
have  had,  in  short,  about  the  same  treatment  as  that  accorded 
to  Amphioxus.  This  group  differs  from  the  latter,  however, 
in  that,  although  isolated,  it  is  extremely  rich  in  the  species 
still  extant,  and  is  represented  by  forms  so  variously  modified, 
and  so  widely  different  from  one  another,  that  they  form  sev- 
eral Orders  and  numerous  Families,  thus  showing  all  possible 
modifications  of  their  fundamental  plan. 

This  group  is  that  of  the  Tunicata,  a  Class  of  marine  ani- 
mals, some  of  which  are  free-swimming  throughout  life,  while 


530 


HISTORY    OF    THE    HUMAN    BODY 


others,  although  active  as  larvae,  soon  settle  to  the  bottom  and 
become  sessile.  Contrary  to  expectation,  it  is  the  latter  which 
in  general  have  retained  the  more  primitive  characteristics, 
while  the  free-swimming  ones  are  often  greatly  modified.  In 
a  typical  sessile  tunicate,  like  the  one  shown  in  Fig.  145,  a, 
the  delicate  and  rather  complex  organism  is  shut  within  a 
tough  and  often  wrinkled  external  coat  or  tunic,  in  which 
there  appear  but  two  definite  structures,  an  incurrent  and  ex- 
current  orifice,  through  which  the  water  is  continually  driven, 


a 


FIG.   145.     Typical  Tunicate. 

(a)    External  view,      (b)    Diagram  of  internal  anatomy. 

IN,  incurrent;  and  EX,  excurrent  canals;  Ph,  pharynx;  INT,  intestine;  Ant 
anus;  CL,  cloacal  chamber;  Tun,  tunic;  Integ,  integument;  End,  endostyle;  GL, 
ganglion;  G,  gonadic  gland;  CD,  gonadic  duct;  GO,  gonadic  opening;  H,  heart. 

much  as  in  sponges.  The  incurrent  orifice,  which  is  really 
the  mouth,  leads  into  a  capacious  pharynx,  and  this  continues 
into  an  intestine  which,  as  usual  in  sessile  forms,  becomes  bent 
upon  itself  and  opens  by  an  anal  orifice  not  far  from  the  phar- 
ynx. The  anus  opens,  not  directly  to  the  exterior,  but  into  a 
cloacal  chamber,  which  is  really  outside  of  the  animal  but  en- 
closed within  the  outer  tunic.  This  same  chamber  surrounds 
the  pharynx  and  receives  the  water  taken  into  the  latter 
through  a  series  of  gill-slits,  which,  in  larvse,  are  few  in  num- 


THE  ANCESTRY  OF  THE  VERTEBRATES   531 

ber  and  disposed  in  lateral  rows,  but  which  usually  become  al- 
most indefinitely  multiplied,  and  in  many  cases  transform  the 
entire  pharyngeal  wall  into  a  structure  resembling  basket-work. 
Thus  the  cloacal  chamber,  except  for  the  relation  of  the  anal 
outlet,  is  precisely  similar  to  the  peribranchial  chamber  of 
Amphioxus,  the  excurrent  orifice  being  the  equivalent  of  the 
atriopore.  The  pharynx  in  the  two  animals  is  also  similar  in 
the  presence  and  secondary  multiplication  of  its  gill-slits,  and 
here  also  there  is  an  endostyle,  which  lies  along  that  side  of  the 
pharynx  which  in  the  free-swimming  larva  is  ventral.  The 
nervous  system  consists  of  a  single  ganglion,  placed  in  the  adult 
between  the  two  arms  of  the  U-shaped  intestine,  but  dorsal 
to  it  in  the  larva,  and  the  vascular  system  is  represented  by 
a  ventrally  placed  heart.  The  reproductive  gonads  lie  in  the 
bend  of  the  intestine,  and  open  by  ducts  of  their  own  into  the 
cloacal  chamber. 

These  numerous  suggestions  of  an  organization  akin  to  that 
of  Amphioxus  which  are  noticeable  in  the  adult  are  far  more 
apparent  in  the  larva  (Fig.  146,  a).  In  this  stage  the  ani- 
mal is  tadpole-like  and  possesses  a  long  tail,  flattened  lat- 
erally and  provided  with  a  typical  notochord,  similar  in  its  de- 
velopment to  that  of  Amphioxus.  A  series  of  segmentally 
arranged  muscles,  separated  by  myocommata,  is  also  found 
here,  especially  developed  posteriorly.  The  nervous  system 
appears  in  the  form  of  a  prolonged  neural  tube  and  expands  at 
its  anterior  end  into  a  sensory  vesicle  (brain)  which  is  pro- 
vided with  a  pigment  speck  that  forms  a  primitive  eye,  and  a 
somewhat  problematic  organ  usually  interpreted  as  an  ear. 
although  both  organs  are  exceedingly  simple  in  construction. 
In  this  larval  condition  the  gill-slits  are  few  in  number  and 
simple  in  arrangement,  and  the  endostyle  is  in  the  proper  po- 
sition for  comparison  with  that  of  Amphioxus.  The  heart 
also  lies  ventrally  and  just  posterior  to  the  oesophagus. 

The  changes  that  take  place  during  the  assumption  of  the 
sessile  position  are  shown  in  Fig.  146,  b  and  c,  and  are  ex- 
plicable by  assuming  a  twist  or  rotation  of  the  animal  towards 
the  left  after  fixation,  by  means  of  two  papillae  of  attachment. 


532  HISTORY   OF   THE   HUMAN   BODY 

a  INT        \         Br 


Br. 


FIG.  146.  Development  of  Tunicate  from  free-swimming  larva.  [After 
SEELIGER.] 

p',  p,  adhesive  suckers;  Br,  brain;  C,  nerve  cord;  INT,  intestine;  H,  heart;  End, 
endostyle;  St,  stolo;  n,  notochord;  my,  myotomes;  t,  tail.  The  orientation  is  in- 
dicated by  arrows,  which  mark  the  incurrent  and  excurrent  orifices. 


THE  ANCESTRY  OF  THE  VERTEBRATES   533 

During  this  metamorphosis,  which  is  a  regressive  one,  the 
tail,  with  its  notochord  and  segmented  muscles,  becomes  lost, 
and  the  central  nervous  system  becomes  much  reduced.  The 
posterior  end  of  the  intestine  connects  with  the  cloacal  cham- 
ber and  the  adult  relationships  are  gained. 

From  this  sketch  of  the  organization  and  metamorphosis 
of  the  tunicates  it  is  evident  that  the  group  is  somewhat  closely 
related  to  Amphioxus,  and  hence  to  the  vertebrates,  but  that, 
since  the  time  of  the  common  ancestor,  the  Tunicata  have  fol- 
lowed for  a  long  distance  a  divergent  road  of  special  adapta- 
tion, which,  although  it  has  allowed  them  to  continue  exist- 
ence, has  been  one  of  degeneracy  and  loss  and  has  masked 
their  true  relationships.  The  ancestor  that  we  here  seek  is 
better  seen  in  the  larva  than  in  the  adult,  and  we  may  believe 
that  there  once  existed  an  adult  animal  with  attributes  like 
that  of  the  tunicate  larva  of  the  present  day,  and  that  this  ani- 
mal was  the  direct  ancestor  of  that  group  of  which  Amphioxus 
is  now  the  only  living  representative. 

Is  there  now  any  record  of  the  history  of  vertebrate  descent 
back  of  the  tunicate  ancestor  ?  Is  there  any  other  invertebrate 
animal  possessed  of  gill-slits,  a  notochord,  and  a  dorsal  nervous 
system  ?  And  as  an  answer  to  this,  a  very  incomplete  and  un- 
certain one  at  best,  there  is  only  a  single  animal  form,  though 
represented  by  several  closely  allied  species,  an  animal  as  un- 
promising in  its  exterior,  and  here,  perhaps,  almost  as  much 
so  in  its  interior  also,  as  those  hitherto  considered.  This 
form  is  Balanoglossus,  a  marine  worm  that  lives  in  self-con- 
structed tubes  of  sand  between  tide-waters,  and,  like  Amphi- 
o.nts,  has  a  wide  distribution.  Externally  the  animal  is  worm- 
like  and  is  possessed  of  four  body  regions,  a  conical  proboscis, 
a  collar  with  a  free  anterior  edge,  a  flattened  gill  region  and  a 
cylindrical  posterior  part.  The  mouth  is  situated  ventrally, 
immediately  beneath  the  edge  of  the  collar,  and  receives  the 
sand  mixed  with  nutrient  material  as  it  becomes  unearthed 
by  the  burrowing  action  of  the  proboscis.  The  pharynx  is 
quite  extended  and  communicates  directly  with  the  exterior 


534 


HISTORY    OF    THE    HUMAN    BODY 


through  two  lateral  rows  of  paired  gill-slits,  which  are  sup- 
ported by  a  branchial  skeleton  very  much  like  that  of  Am- 
phwxus. 

This  characteristic  of  the  possession  of  pharyngeal  gill- 
slits  is  extremely  significant,  for  nowhere  else,  except  in  ver- 
tebrates and  in  their  allies,  above  considered,  do  such  organs 
occur,  and  investigators  have  naturally  been  led  through  these 
to  seek  for  the  other  essential  vertebrate  characters  of  a  noto- 


FIG.   147.     Balanoglossus.     [After  Loos,  in  LEUCKART  charts.] 

chord  and  a  dorsal  nervous  system.  In  this  search  they  have 
been  to  a  qualified  extent  successful,  although  these  characters 
are  far  from  appearing  with  the  same  distinctness  as  in  the 
case  of  the  gill-slits.  The  nervous  system  consists  in  general 
of  a  diffuse  net-work  of  nerve  fibers  lying  in  the  depth  of  the 
surface  epithelium  and  occurring  everywhere,  a  very  low  type 
of  nervous  system.  This  net-work  is,  however,  reinforced  and 


THE  ANCESTRY  OF  >HE  VERTEBRATES  535 

formed  into  four  longitudinal  cords,  slightly  differentiated 
from  the  rest,  a  dorsal,  a  ventral,  and  two  lateral,  all  of  which 
run  the  entire  length  of  the  animal.  Of  these  the  dorsal  re- 
ceives slightly  more  emphasis  than  the  others,  since  it  continues 
forward  to  the  base  of  the  proboscis,  where  it  divides  into 
two  diverging  branches,  which  encircle  it  in  the  form  of  a 
ring.  As  for  the  notochord,  this  has  been  doubtfully  identified 
with  a  small  diverticulum,  which  arises  from  the  dorsal  wall 
of  the  pharynx,  and  extends  some  distance  forward  into  the 
proboscis,  and  this  supposition  has  been  greatly  strengthened 
through  the  recent  discovery  of  an  allied  form  belonging  to 
a  new  genus  (Harrimania)  in  which  the  diverticulum  is  much 


es-  Lro 

FIG.  148.     Harrimania  maculosa.     [After  RITTER.] 

Schematic  representation  of  dissection,  including  collar  and  small  portion  of 
the  anterior  pharyngeal  region.  The  anterior  and  posterior  aspects  are  designated 
as  A  and  P,  respectively,  es,  oesophagus;  es.  No-c,  cesophageal  notochord;  d.  n.  c, 
dorsal  nerve  cord;  SK.  C,  skeletal  crura;  br.  o,  branchial  orifices. 

larger,  and  in  its  mode  of  origin  is  strikingly  similar  to  that 
of  the  true  vertebrate  notochord,  and  is  thus  without  much 
doubt  homologous  with  this  organ. 

From  the  testimony  afforded  by  the  structure  of  Balanoglos- 
sus  and  its  allied  genera  (the  group  Enteropneusta)  it  may  be 
quite  confidently  asserted  that  these  forms  lie  nearly  in  the  line 
of  vertebrate  descent,  and  represent  an  earlier  stage  than  that 
of  the  tunicates.  But  here  the  chain  seems  to  end,  for  Balano- 
glossus  is  itself  unusually  isolated  and  shows  no  close  affinity 
to  any  other  invertebrate  types.  There  is,  in  such  cases,  but 


536 


HISTORY    OF    THE    HUMAN    BODY 


one  possible  way  out,  a  single  remaining  clew,  and  that  is, 
the  embryology  of  the  form  in  question,  and  even  here  the 
primary,  historic  features  may  be  overlaid  with  secondary 
changes  rendered  necessary  as  an  adaptation,  and  thus  the 
value  of  a  given  feature  is  often  hard  to  estimate.  In  the 
case  of  Balanoglossus,  however,  it  seems  probable  that  the  early 
development  is  in  great  part  an  actual  repetition  of  the  race- 
history,  but  if  so,  it  leads  us  to  surprising  and  not  very  satis- 
fying results,  for  the  animal  begins  life  as  a  minute  transparent 
floating  larva,  the  Tornaria-)  furnished  with  bands  of  cilia,  by 
which  it  moves,  a  larva  strikingly  like  that  of  star-fish,  sea- 
urchins,  and  other  echinoderms,  and  one  which  an  unprejudiced 


A 


B 


FIG.  149.  Comparison  of  Tornaria  larva  with  larval  Echinoderms. 
[After  O.  HAMANN.]  Main  ciliated  bands  in  black,  lesser  systems  cross- 
lined. 

(A)  Tornaria,  ventral  view.  (B)  Tornaria,  dorsal  view.  (C)  Auricularia,  ven- 
tral view.  (D)  Bipinnaria,  ventral  view. 

mind  would  not  hesitate  to  classify  with  these  latter.  These 
larvae  are  all  of  about  the  same  size,  all  bilateral  in  structure,  all 
transparent  and  equipped  with  bands  of  cilia,  and  there  is  even 
a  close  correspondence  in  the  manner  of  disposal  of  these 
bands. 

In  the  case  of  the  echinoderms  the  universal  occurrence  of 
such  larvae  is  taken  everywhere  as  a  proof  that  they  represent 
an  early  stage  in  the  history  of  the  Class,  and  that  the  ancestors 
of  these  radiate,  crawling,  or  sessile,  bottom  forms  were  bi- 
lateral and  pelagic.  Now  it  would  be  highly  improbable  that 


THE  ANCESTRY  OF  THE  VERTEBRATES  537 

an  unrelated  form  should,  as  an  adaptation,  so  modify  its 
early  stages  as  to  resemble  these  echinoderm  larvae  as  closely 
as  does  the  Tornaria,  and  the  only  alternative  is  to  accept  as  a 
very  ancient  common  ancestor  of  both  echinoderms  and  verte- 
brates the  form  which  all  these  larvce  may  be  said  to  copy;  a 
form  having  the  characteristics  common  to  all,  including  bi- 
laterally, minute  size,  transparency,  locomotion  by  bands  of 
cilia,  and  pelagic  life.  The  lineal  descendants  of  this  hypotheti- 
cal ancestor  chose  two  paths,  the  one  leading  to  the  Echino- 
dermata,  the  other  to  Balanoglossus,  the  Tunicata,  Amphi- 
oxus,  and  eventually  the  Vertebrata. 

This    theory,   although    incomplete    and    unsatisfactory    in 


B 


FIG.    150.     Comparison    of    Tornaria    and    Echinoderm    larvae,    lateral 
views.     [After  BALFOUR.] 

(A)    Tornaria.      (B)    Auricularia.      (C)    Bipinnaria. 

a,   apical    area;    b,    oral    area;   c,   post-oral    area;    d,    anal   area. 

parts,  is  consistent  with  the  most  approved  lines  of  biological 
thought;  it  rests  upon  development  as  well  as  adult  structure, 
and  bears  the  indorsement  of  the  majority  of  investigators 
at  the  present  time.  The  weakest  part  of  the  argument  is  that  of 
the  significance  of  the  Tornaria  larva ;  and  while  the  acceptance 
of  this  gives  us  very  little  enlightenment,  to  abandon  it  would 
be  to  sacrifice  but  little,  and  would  render  the  gulf  between 
the  adult  Balanoglossus  and  other  invertebrates  only  a  little 
more  profound.  To  summarize  in  the  words  of  two  recent 


538  HISTORY   OF   THE    HUMAN    BODY 

authors,*  "  The  question  of  the  descent  of  the  Chordata  is  not 
solved  by  accepting  their  relationship  to  the  Enteropneusta, 
since  this  latter  group  holds  an  uncommonly  isolated  position. 
Only  from  the  structure  of  the  Balanoglossus  larva  can  there 
be  concluded  a  distant  connection  with  the  echinoderms.  We 
must  resign  ourselves  to  the  thought  that  at  the  present  time 
we  are  not  in  a  condition  to  assert  from  what  ancestral  form  the 
Chordata,  and  with  them  Balanoglossus,  are  to  be  derived. 
The  origin  of  the  vertebrates  is  lost  in  the  obscurity  of  forms 
unknown  to  us." 


*  Korschelt  u.  Heider.  Entwickelungsgeschichte.  Jena,  1893,  p.  1465. 
"Die  Frage  nach  der  Abstammung  der  Chordaten  wird  durch  die  An- 
nahme  von  verwandtschaftlichen  Beziehungen  derselben  zu  den  Enterop- 
neusten  nicht  gelost,  da  die  letzere  Gruppe  selbst  ungemein  isolirt  dasteht. 
Nur  aus  dem  Bau  der  Balanoglossuslarve  lasst  sich  eine  entferntere 
Zusammenhang  mit  den  Echinodermen  erschliessen.  Wir  miissen  uns 
bei  den  Gedanken  resigniren,  dass  wir  vorlaufig  nicht  im  Stande  sind, 
anzugeben,  von  welchen  Urformen  die  Chordaten  und  mit  ihnen  Balan- 
oglossus herzuleiten  sind.  Der  Ursprung  der  Wirbelthiere  verliert  sich 
in  das  Dunkel  uns  unbekannter  Formen." 


- 


APPENDIX 

CLASSIFICATION  OF  THE  VERTEBRATA. 

The  following  list  of  the  larger  subdivisions  of  the  Vertebrata, 
arranged  in  synoptical  form,  may  be  of  use  in  explaining  the 
names  of  groups  as  used  in  the  body  of  the  work.  Through 
the  labors  of  palaeontologists  so  many  forms  have  been  unearthed 
and  so  many  new  groups  established  that  it  seems  best  to  include 
in  the  list  these  latter  as  well  as  modern  animals,  especially  since 
many  of  the  extinct  groups  consist  of  generalized  forms  from 
which  several  living  groups  have  differentiated.  The  names  of  all 
groups,  of  whatever  rank,  that  contain  living  representatives  are 
printed  in  bold-faced  type;  those  groups  in  which  these  latter 
are  represented  by  but  one  or  two  isolated  forms  are  farther 
designated  with  an  asterisk.  In  this  way  the  amount  of  dam- 
age wrought  in  the  phylogenetic  record,  as  well  as  the  relative 
position  of  the  groups,  may  be  seen  at  a  glance.  The  synopsis 
follows : — 

VERTEBRATA  (or  Chordata). 
Division  I.  Cyclostomata.* 

Class  I.  Marsipobranchii.* 
Sub-Class  I.  Cyclostomi.* 

Order  I.  Myxinoidea  *   (Myxine,  the  hag-fish). 
Order  2.  Petromyzontoidea  *       (Petromyzon,      the 
lamprey  eel). 

Sub-Class  II.  Ostracodermi. 

Order  i.  Heterostraci  (Pteraspis). 
'  Order  2.  Osteostraci  (Cephalaspis). 

Order  3.  Antiarchi  (Pterichthys). 

539 


540          HISTORY  OF  THE  HUMAN  BODY 

Division  II.  Gnathostomata. 
Super-Class  I.  Ichthyoidea. 
Class  I.  Pisces. 

Sub-Class  I.  Elasmobranchii. 

Order  i.  Pleuropterygii   (Cladoselache) . 
Order  2.  Ichthyotomi  (P I eur acanthus). 
Order  3.  Acanthodii  (A  cant  hod  es,  Diplacanthus). 
Order  4.  Selachii. 

Sub-Order  I.  Squall  (sharks,  dog-fish). 
Sub-Order  2.  Raiae  (skates). 
Sub-Class  II.  Holocephali.* 

Order  i.  Chimaeroidei  *  (Chimcera). 
Sub-Class  III.  Dipnoi.* 

Order  i.  Sirenoidei  *  (Protopterus,  Ceratodus). 
Order  2.  Arthrodira  (Coccosteus,  Dinichthys). 
Sub-Class  IV.  Teleostomi. 
Order  i.  Crossopterygii.* 

Sub-Order  i.  Haplistia  (Tarrasius). 

Sub-Order  2.  Rhipidistia      (Holoptychius;      Oste- 

olepis). 

Sub-Order  3.  Actinistia  (Ccelacanthus ;  Undina). 
Sub-Order  4.  Polypteroidei  *    (Polypterus;    Cala- 

moichthys). 
Order  2.  Actinopterygii. 

Sub-Order  i.  Chondrostei  *     (Palaoniscus  ;    Aci- 

penser,  sturgeon). 
Sub-Order  2.  Protospondyli  *      (Lepidottts;     Eu- 

gnathus;  Amia,  bow-fin). 
Sub-Order  3.  ^Etheospondyli  *    (Aspidorhynchiis; 

Lepisosteus,  gar-pike). 
Sub-Order  4.  Isospondyli     (Leptolepis;    herring; 

salmon;  trout). 
Sub-Order  5.  Eventognathi     (carp ;     minnow; 

sucker). 

Sub-Order  6.  Nematognathi    (siluroids,   e.g.  bull- 
heads, cat-fish,  etc.). 
Sub-Order  7.  Haplomi    (pike;  killifish). 
Sub-Order  8.  Apodes   (eels). 
Sub-Order  9.  Synentognathi     (flying-fish). 


APPENDIX  541 

Sub-Order  10.  Lophobranchii  (sea-horse;  pipe- 
fish). 

Sub-Order  n.  Hemibranchii  (stickleback). 

Sub-Order  12.  Acanthopteri  (mackerel;  cod; 
perch;  sculpin;  flounder). 

Sub-Order  13.  Pediculati   (angler-fish;  frog-fish). 

[In  the  Class  of  Pisces  the  process  of  extinction  has  left 
in  our  modern  fauna  five  more  or  less  isolated  groups, 
usually  treated  as  Orders.  These  are  the  Selachii,  Holo- 
cephali,  Dipnoi,  Ganoidei  and  Teleostei.  The  first  is  the 
only  remaining  group  of  the  elasmobranchSj  of  the  second 
and  third  the  only  fossils  known  are  much  like  those  of 
the  present  day  and  their  affinities  have  thus  not  been 
definitely  traced ;  and  the  fourth  and  fifth  are  respectively 
the  earlier  and  later  types  of  teleostomes. 

The  living  ganoids  are  very  few  in  number  and  are 
for  the  most  part  unrelated  to  one  another.  There  are 
but  two  crossoptergyians,  and  but  one  or  two  living 
genera  of  each  of  the  three  groups  Chondrostei,  Protospon- 
dyli,  and  sEtheospondyli.  At  this  point  the  "  ganoids " 
are  considered  to  end,  and  the  remaining  Sub-orders,  be- 
ginning with  the  Isospondyli,  are  included  with  the  tele- 
osts  (i.  e.  Teleostei,  to  be  carefully  distinguished  from 
Teleostomi,  the  larger  group). 

Sub-orders  4-8  are  sometimes  grouped  as  the  Physostomi 
and  the  remaining  sub-orders,  5-13,  as  the  Physoclysti. 
In  the  former  of  these  the  air-bladder  retains  its  connec- 
tion with  the  alimentary  canal;  in  the  latter  this  becomes 
lost  during  development  and  the  air-bladder  is  a  closed 
sac.] 

Class  II.  Amphibia  (Batrachia). 
Order  i.  Urodela. 

Sub-Order  i.  Perennibranchiata    (Necturus;    Si- 
ren). 

Sub-Order  2.  Derotremata  (Cryptobranchus). 
Sub-Order  3.  Salamandrida       (newts;      salaman- 
ders). 

Order  2.  Gymnophiona   (subterranean  forms,  with- 
out limbs  or  eyes). 
Order  3.  Anura. 

Sub-Order  i.  Aglossa  *   (Surinam  toad). 
Sub-Order  2.  Arcifera  (toads;  tree-toads). 
Sub-Order  3.  Firmisternia   (frogs). 


542 


HISTORY  OF  THE  HUMAN  BODY 

Order  4.  Stegocephali. 

Sub-Order  I.  Branchiosauria    (Branchiosaurus) . 

Sub-Order  2.  Aistopoda  (snake-like  forms,  with- 
out limbs). 

Sub- Order  3.  Microsauria  (small  forms,  in  shape 
like  salamanders). 

Sub-Order  4.  Labyrinthodontia  (Archcegosaurus, 
Mastodonsaurus) . 

[Of  the  four  Orders  of  Amphibia,  one  is  entirely  extinct 
and  the  other  three  essentially  modern,  and  with  few 
traces  of  older  representatives.  With  regard  to  the  ex- 
tinct group,  that  of  Stegocephali,  it  could  be  placed  in 
the  list  either  at  the  first  or  the  last,  since  it  shows  strong 
affinities  to  both  ganoids  and  reptiles  and  thus  lies  inter- 
mediate between  the  two.  The  modern  Orders  seem  to 
have  arisen  directly  from  the  Stegocephali,  the  Urodela 
being  the  least  altered  and  hence  the  most  important 
morphologically.  This  complex  relationship  between  the 
groups  mentioned,  the  Ganoidei,  Stegocephali,  Urodela, 
Reptilia,  etc.,  cannot  thus  be  represented  in  linear  lines 
but  may  be  partly  expressed  in  the  form  of  a  tree,  as  in 
the  diagram  given  in  Chapter  II.  (p.  28).  The  three  modern 
Orders  of  Urodela,  Gymnophiona,  and  Anura,  are  quite 
distinct  from  one  another,  and,  the  first  and  third  espe- 
cially, are  well  represented  in  the  living  fauna.] 

Super-Class  II.  Sauropsida. 
Class  III.  Reptilia. 

Order  i.  Theromorpha. 

Sub-Order  i.  Pariasauria   (Pariasaurus). 
Sub-Order  2.  Theriodontia    (Cynognathus;    Trity- 

lodon). 
Sub-Order  3.  Dicynodontia     (Dicynodon;    Gordo- 

nia). 
Order  2.  Sauropterygia     (Pleisiosaurus;     Cryptocli- 

dus). 
Order  3.  Chelonia. 

Sub-Order  i.  Cryptodira    (the  majority  of  living 

turtles). 

Sub-Order  2.  Pleurodira  (Miolania;  Chelys). 
Sub-Order  3.  Trionychia  (soft-shelled  turtles). 
Order  4.  Ichthyopterygia  (Ichthyosaurus). 


APPENDIX  543 

Order  5.  Rhynchocephalia.* 

Sub-Order  i.  Proterosauria    (Proterosaurus ;    Pa- 

Iceohatteria) . 

Sub-Order  2.  Rhynchocephalia    vera  *     (Sphe no- 
don    [Hatteria],    the    only    living 
representative  of  the  Order). 
Order  6.  Squamata. 

Sub-Order  I.  Dolichosauria  (Dolichosaurus). 
Sub-Order  2.  Pythonomorpha  (Mosasaurus). 
Sub-Order  3.  Lacertilia   (lizards). 
Sub-Order  4.  Ophidia  (snakes). 
Order  7.  Dinosauria. 

Sub-Order  I.  Theropoda  (Anchisaurus) . 
Sub-Order  2.  Sauropoda  (Brontosaurus). 
Sub-Order  3.  Ornithopoda      (Iguanodon;     Stego- 

saurus). 
Order  8.  Crocodilia.* 

Sub-Order  i.  Parasuchia  (Belodon). 

Sub-Order  2.  Mesosuchia     (Pelagosaurus;    Teleo- 

saurus) . 
Sub-Order  3.  Eusuchia  *   (Thoracosaurus ;  Croco- 

dilus;  Alligator). 

Order  9.  Pterosauria    (Pterodactylus;  Rhamporhyn- 
chus). 

[As  is  indicated  above  by  the  difference  in  type  the  proc- 
ess of  extinction  in  the  group  of  Reptilia  has  gone  very 
far,  leaving  but  four  isolated  spots  to  be  represented  among 
the  living  forms;  (i)  the  Chelonia,  essentially  a  modern 
group,  (2)  the  Rhynchocephalia,  represented  by  a  single 
living  species,  (3)  the  last  two  Sub-Orders  of  the  Squamata, 
the  lizards  and  snakes,  and  (4)  a  very  few  modern  representa- 
tives or  the  Crocodilia.  In  arrangements  in  which  living  forms 
are  alone  taken  into  consideration,  these  are  given  as  five 
Orders;  the  lizards  and  snakes  count  as  two;  in  the  earlier 
works  Sphenodon  was  counted  among  the  lizards,  redr.cing 
the  number  to  four.  The  Orders,  as  thus  arranged,  are  as 
follows :  Chelonia,  Rhynchocephalia,  Lacertilia  f  Ophidia, 
Crocodilia.} 

Class  IV.  Aves. 

Order  i.  Saururae  (Arch&opteryx;  Laopteryx). 
Order  2.  Odontormae  (Ichthyornis;  Apatornis). 


544          HISTORY  OF  THE  HUMAN  BODY 

Order  3.  Odontoholcse  (Hesperornis;  Lestornis). 
Order  4.  Eurhipidurae. 

Sub-Order  i.  Dromaeognathi. 

Section  I.  Struthiones  (ostrich,  casuary). 
Section  II.  Aepiornithes  (^piornis). 
Section  III.  Apteryges  *   (Apteryx). 
Section  IV.  Crypturi  (tinamoo,  Crypturus). 
Section  V.  Gastornithes   (Gastornis). 
Sub-Order  2.  Impennes  (penguins). 
Sub-Order  3.  Euornithes   [a  recent  group,  begin- 
ning in  the  Eocene], 

Section  I.  Desmognathae  (ducks;  herons; 
eagles;  hawks;  owls;  cuckoos; 
kingfisher ;  trogon ;  parrot) . 

Section  II.  Schizognathae  (grebes;  loons;  gulls; 
snipes;    grouse;    quails;   pigeons; 
humming-birds ;  woodpeckers ) . 
Section  III.  .^Egithognathae   (Passeres,  a  group 
which    includes   over   one-half   of 
the  species  of  living  birds). 
Super-Class  III.  Mammalia. 
Class  V.  Mammalia. 

Sub-Class  I.  Prototheria  *  [Ornithodelphia]. 

Order  i.  Pantotheria  (Dromotherium;  Amphilestes). 
Order  2.  Multituberculata     (Ctenadon;     Polymasto- 

don). 

Order  3.  Monotremata  *    (Ornithorhynchus;  Echid- 
na). 

Sub-Class  II.  Eutheria. 
Super-Order  i.  Didelphia  [Marsupialia]. 
Order  I.  Polyprotodontia  (opossum;  Thylacinus). 
Order  2.  Paucituberculata  *    (Ccenolestes). 
Order  3.  Diprotodontia    (Petaurus;    wombat;    kan- 
garoo). 

Super-Order  2.  Monodelphia  [Placentalia]. 
Order  i.  Insectivora  (moles;  shrews;  hedgehog). 
Order  2.  Cheiroptera  (bats). 
Order  3.  Galeopithecidae  *   (flying  lemur). 
Order  4.  Edentata. 


APPENDIX  545 

Sub-Order  I.  Tubulidentata  *    (Orycteropus,    the 

earth-hog  or  "  aard-vark."). 
Sub-Order  2.  Pholidota  *  (Manis,  a  scaled  animal 

of  Asia  and  Africa). 

Sub-Order  3.  Xenarthra     (Glyptodon;    Megathe- 
rium;   Grypotherium;   ant-eaters; 
armadilloes;  sloths). 
Order  5.  Rodentia. 

Sub-Order  i.  Tillodontia   ( Tillotherium;  Es- 

thonyx) . 

Sub-Order  2.  Duplicidentata  (hares;  rabbits). 
Sub-Order  3.  Simplicidentata  (squirrels;  beavers; 

mice). 
Order  6.  Primates. 

Sub-Order  i.  Mesodonta     (Adapis;     Anaptomor- 

phus). 
Sub-Order  2.  Lemuroidea     (Lemurs;     Chiromys; 

Tar  si  us). 

Sub-Order  3.  Anthropoidea. 
Division  i.  Platyrrhini     (Hapale;    Midas;    Ce- 

bus). 
Division  2.  Catarrhini    (Cercopithecus;  Semno- 

pithecus;  Gorilla;  Homo). 
Order  7.  Creodonta  (Arctocyon;  Hyanodon). 
Order  8.  Carnivora  (cats;  dogs;  bear;  weasel). 
Order  9.  Pinnipedia  (seals;  walrus;  sea-lion). 
Order  10.  Cetacea   (whales;  porpoises). 
Order  n.  Condylarthra  (Phenacodus). 
Order  12.  Hyracoidea  *  (Procavia  [Hyrax]). 
Order  13.  Amblypoda   (Coryphodon). 
Order  14.  Sirenia  *   (manatee;  dugong). 
Order  15.  Proboscidea  *  (Mastodon;  elephants). 
Order  16.  Ancylopoda   (Homalodontotherium). 
Order  17.  Typotheria  (Typotherium). 
Order  18.  Toxodontia  (Toxodon). 
Order  19.  Litopterna  (Proterotherium). 
Order  20.  Perissodactyla     (Palaotherium;     Titano- 

therium;  rhinoceros;  horse). 
Order  21.  Artiodactyla. 


546          HISTORY  OF  THE  HUMAN  BODY 

Sub-Order  i.  Suina  (Elotherium;  pigs;  peccaries; 
hippopotamus). 

Sub-Order  2.  Tylopoda  (Oreodon;  camel;  llama). 

Sub-Order  3.  Anthracotherioidea  (Anthrac  other  i- 
um). 

Sub-Order  4.  Dichobunoidea  (Dichobune;  Ano- 
plotherium). 

Sub-Order  5.  Traguloidea  (Tragulus,  several 
small  species  in  E.  Indies). 

Sub-Order  6.  Pecora  (deer;  sheep;  cattle;  gi- 
raffe). 

[The  arrangement  of  animal  groups  in  the  form  of  a 
list,  in  which  they  follow  one  another  in  a  single  series, 
is  seldom  more  unsatisfactory  than  it  is  in  the  case  of 
mammals.  The  inadequacy  of  this  method  in  expressing 
the  true  relationships  is  seen  if  the  list  be  compared  with 
the  phylogenetic  tree  given  in  Chapter  II.  (p.  36).  There 
are  several  distinct  stems  to  be  followed  and  the  order 
in  which  they  are  taken  in  a  list  is  largely  a  matter  of 
preference.  Here  the  attempt  is  made  to  proceed  from 
the  generalized  to  the  more  specialized  ones,  and  thus 
the  main  stem  of  the  Inscctivora  is  taken  first ;  then  that 
of  the  Primates,  and  lastly  the  complex  and  highly  spe- 
cialized branch  leading-  to  the  carnivore  and  ungulate 
Orders.  This  arrangement  has  the  advantage  of  empha- 
sizing the  primitive  and  rather  generalized  structure  of 
the  Primates  as  compared  with  the  groups  just  men- 
tioned, a  comparison  entirely  lost  sight  of  by  the  usual 
arrangement,  which  places  the  apes  and  man  at  the  top. 
If  the  arrangement  be  made  solely  on  the  basis  of  the 
development  of  the  nervous  system  there  can  be  no  ques- 
tion of  the  rightfulness  of  this  position;  but  if  all  the 
systems  be  taken  into  consideration,  and  especially  the 
bones,  muscles  and  teeth,  which  in  other  groups  form  the 
principal  criteria  for  the  purpose  of  classification,  the 
Primates  are  found  to  have  retained  a  larger  number  of 
primitive  characters  than  any  other  placenta!  group  with 
the  exception  of  the  Insectivora,  Rodentia,  and  Edentata, 
and  thus  to  stand  far  loivcr  in  the  scale  of  specialisation 
than  the  manifold  descendants  of  the  Creodonta  and 
Condylarthra. 

The  arrangement  of  the  subdivisions  of  the  Primates 
given  above  is  a  conservative  one,  and  will  accord  with 
the  most  of  the  literature  on  the  subject.  Certain  im- 
portant modifications  have,  however,  been  recently  pro- 


APPENDIX 


547 


posed,  based  upon  a  more  complete  study  of  anatomical 
characters.  Through  these  the  extinct  genus  Anaptomor- 
phus,  which  is  probably  very  near  the  direct  ancestral 
line  leading  to  Man,  has  been  placed  in  Sub-order  3, 
Anthropoidea,  and  with  it  has  been  placed  the  living  genus, 
Tarsius,  a  closely  related  form. 

For  convenience  in  classification  all  the  descendants  of 
the  Condylarthra,  together  with  this  latter,  but  excepting 
the  aberrant  Sirenia,  are  often  grouped  together  under 
the  single  Order  of  Ungulata,  or  hoofed  animals,  connected 
both  by  descent  and  by  the  common  peculiarities  embodied 
in  the  name.  This  will  include  Orders  11-21  in  the  above 
list.  In  the  same  way  the  Creodonta  may  be  included  in 
the  Carnivora,  although  the  Cetacea  are  treated  as  a  sepa- 
rate, though  allied  group.  The  Galeopithecoidea  are  often 
included  within  the  Insectivora.  This  reduces  the  Orders 
of  placenta!  mammals  to  nine,  viz :  Insectivora,  Cheiroptera, 
Edentata,  Rodentia,  Primates,  Ungulata,  Sirenia,  Cetacea, 
Carnivora.  There  are,  of  course,  as  in  all  groups  of  ani- 
mals, many  other  possible  arrangements,  the  differences 
being  based  on  the  relative  value  of  the  various  groups, 
their  relationships  to  one  another,  and  the  comparative  de- 
gree of  specialization  of  each ;  points  upon  which  there  is 
„  much  room  for  difference  of  opinion.] 

Owing  to  the  extinction  of  so  many  of  the  groups,  especially 
those  forming  the  connection  between  two  others,  a  classification 
that  rests  wholly  upon  living  forms  is  far  from  complete  and  in 
some  points  differently  arranged  from  one  that  includes  all 
known  forms.  Thus  among  the  fishes  the  selachians  alone  are 
left  of  all  the  elasmobranchs ;  a  few  remnants  remain  of  the  first 
few  Orders  of  teleostomes,  isolated  from  one  another  and  from 
the  others ;  and  of  the  Holocephali  and  Dipnoi  only  a  few  species 
occur.  Among  the  amphibians,  the  Stegocephali,  the  most  im- 
portant Order  of  all,  have  disappeared  entirely,  and  among  the 
reptiles  a  still  greater  destruction  has  left  but  four  isolated  spots 
in  a  once  continuous  history.  This  loss  has  affected  also  all  the 
transition  forms  between  reptiles  and  the  modern  type  of  birds, 
and  completely  isolated  the  Aves  from  all  related  forms.  The 
mammals  are  still  rich  in  Orders  but  the  synthetic  types  that  once 
united  them  have  long  since  disappeared.  Without  going  into 
the  Sub-Orders  this  abbreviated  classification,  in  some  respects 
different  from  the  above  synopsis,  may  be  given  here  for  con- 
venience in  comparison.  Only  the  gnathostomes  may  be  con- 
sidered. 


548          HISTORY  OF  THE  HUMAN  BODY 

SYNOPSIS  OF  VERTEBRATA  (living  forms  alone). 

Class  I.  Pisces. 

Sub-Class  I.  Selachii. 

Sub-Class  II.  Holocephali    (often  considered  with  the 

previous  group). 
Sub-Class  III.  Ganoidei. 
Sub-Class  IV.  Teleostei. 
Sub-Class  V.  Dipnoi. 
Class  II.  Amphibia. 
Order  i.  Urodela. 
Order  2.  Gymnophiona. 
Order  3.  Anura. 
Class  III.  Reptilia. 

Order  i.  Chelonia. 

Order  2.  Lacertilia  (including  Sphenodon). 
Order  3.  Ophidia. 
Order  4.  Crocodilia. 
Class  IV.  Aves. 

Sub-Class  I.  Ratitse    (running  birds;  with  flat  breast- 
bone, e.g.  Ostrich). 
Sub-Class  II.  Carinatse     (flying     birds;     with     keeled 

breast-bone). 
Class  V.  Mammalia. 

Sub-Class  I.  Prototheria. 
Order  i.  Monotremata. 
Sub-Class  II.  Eutheria. 
Super-Order  i.  Didelphia  (Marsupialia). 
Super-Order  2.  Monodelphia  (Placentalia). 
Order  i.  Edentata. 
Order  2.  Insectivora. 
Order  3.  Rodentia. 
Order  4.  Cetacea. 
Order  5.  Sirenia. 
Order  6.  Ungulata. 
Order  6a.  Proboscidea   (occasionally  separated  from 

the  Ungulata). 

Order  6b.  Hyracoidea    (occasionally   separated   from 
the  Ungulata). 


APPENDIX  549 

Order  7.  Carnivora. 
Order  8.  Cheiroptera. 
Order  9.  Primates. 

In  this  arrangement  there  will  be  noticed  especially  the  separa- 
tion of  ganoids  and  teleosts,  the  small  number  of  reptilian  Or- 
ders, the  complete  isolation  of  the  birds,  and  the  arrangement  of 
the  mammalian  Orders  in  such  a  way  as  to  bring  the  Primates  at 
the  top.  Certain  of  these  faults,  like  the  isolation  of  the  birds, 
have  been  corrected  for  some  time,  the  separation  of  ganoids  and 
teleosts  is  a  convenient  one  for  purposes  of  comparative  anat- 
omy, and  is  employed  for  this  purpose  in  the  body  of  this  work. 
The  arrangement  of  the  mammals  to  show  the  supremacy  of 
Man  is  natural,  and  is  based,  of  course,  in  part,  on  the  high  de- 
velopment of  the  brain,  but  much  is  due  to  natural  human  pride 
which  recognizes  man's  mental  supremacy,  and  feels  that  a  su- 
premacy in  physical  structure  must  also  be  granted.  As  a  matter 
of  fact,  in  all  other  aspects  save  that  of  the  brain,  the  apes  and 
man  are  rather  primitive  in  their  structure  and  show  a  far  less 
bodily  specialization  than  almost  any  of  the  other  living  Orders  of 
mammals,  the  Insectivora  and  a  few  others  being  alone  excepted. 


INDEX 


(In    "using    this     index    consult,  for   a  given   animal,   both  the   sci- 
entific  and   common   names.) 


abdominal    ribs,   141 
abomasus,   292 
achselbogen,    251 
acinous   glands,  97,   112 
Acipenser,    171 
acoelous   vertebrae,    131 
acoustic    hairs,    467 
acoustic   maculae,  409 
adenoid    tissue,    362 
afferent    nerves,    408,    435 
air-bladder,  270,  310,  311 
air-cells,  314 
albatross,   478 
alimentary    canal,     258,     259, 

261,  262,  263,  264,  265,  266, 

299,   365 

alisphenoids,  148 
allantoic   arteries,   70,  71 
allantoic  veins,  70,  71 
allantois,  70,  71,  322,  323,  324, 

377,  378 

alveoli,  of  jaws,  273 

alveoli,   of  lungs,  314 

Amblypoda,  38 

Ammocoetes,   477 

Ammon's   horn,  417 

amnion,  70,  71 

Amniota,   19,  70,  78,   174,  373, 

378,  383,   384,   385,  393,  427, 
amniotic   fluid,  70,   71 
amphibians,  no,   148,  166,  170, 

174,  178,  200,  202,  203,  220, 
239,  245,  253,  271,  272,  283, 
288,  291,  294,  296,  308,  310, 
322,  328,  331,  344,  346,  347, 
353,  356,  358,  369,  375,  383, 
392,  398,  409,  415,  416,  419, 
427,  439,  444,  456,  464,  465, 


amphibians — (Continued) 
472,  479,  484,  488,  489,  491,  504, 
5i8 

amphiccelous  vertebrae,  127 
Amphioxus,  24,  26,  27,  28,  29,  44, 
59,   61,  63,  66,  67,   76,   123,    143, 
152,  154,   162,   163,  200,  260,  267, 
289,  290,  303,  305,  306,  353,  365, 
411,  432,  436,  443,  445,  460,  461, 
476,  503,  515,  517,  521,  525,  526, 
527,  528,  529,  531,  533,  534,  537 
amphirrhine   condition,   476 
Amphiuma,  305,  313 
amplexation,  383,  397 
ampullae,  of  semicircular  canals,  487 
ampullae,  of  slime  canals,  471 
260,      anal  fin,   164 
298,      anal   sacs,    113 

Anamnia,  70,  78,  384 
Anarrhichas,    489 
Ancylopoda,  38 
angulare,    156 
347,      annelids,    368,    460,    512,    513,    514, 

515,  517,  518,  519,  520,  524 
annelid  theory  of  vertebrate  ances- 
try, 513,  520 
ansa  hypoglossi,  458 
ant-eater,    316,    393 
anterior   girdle,    130 
anthropoids,  38,  41,  43,  45,  90,  214, 
375,          226,  230,  251,  256,  298,  409,  447, 
457          480,  496,  500,  504 

antitropists,  244 

172,       antrnm    of    Highmore,    482 
237,      Anura,  31,  220,  313,  355,  486,  493, 
285,          495 

319,      anus,  258,  259,  296 
352,       aorta,  aortse,  320,  326,  339,  351,  354 
385,      apes,    45,    214,    223,    229,    253,    297, 
425,          390 
469,       apical  pads,  91 

551 


552 


INDEX 


appendages,  510 

appendicular  muscles,  190,  192,  193 

appendictilar    skeleton,    122,    162 

appendix,  297,  298 

appendix   testis,   392,   394 

aqueductus    cerebri,    412 

aqueductus    Sylvii,    412 

aqueductus    vestibuli,   486 

aqueous  humor,  of  eye,  501 

arachnoids,  522,  "524 

arachnoid  theory  of  vertebrate  an- 
cestry, 522,  525 

Archaeopteryx,    19,  32 

archencephalon,  411,  443,  527 

archetype,  500,  509 

archipterygium,    176,    185 

archisternum,    138,    141 

area  centralis,  of  retina,  499 

armadillo,  85,   393 

Artemia,  55 

arterial  arches,  320,  321,  328-339 

artery,  or  arteries: — 318,  328;  al- 
lantoic,  322;  anonyma,  332; 
aorta,  aortse,  320,  326,  339,  351, 
354;  aortic  arch,  331,  356;  affer- 
ent branchials,  325,  329;  arterial 
arches,  320,  321,  328;  branchials, 
afferent,  325,  329;  branchials, 
efferent,  326;  carotid,  321,  326, 
332-339;  carotis  cerebralis,  333, 
338;  caudal,  339;  common 
carotid,  338;  ductus  arteriosus, 
331 ;  ductus  botalli,  331 ;  efferent 
branchials,  326;  external  carotid, 
338;  hypoglossal,  335;  iliac,  322, 
326,  339;  innominata,  332;  in- 
fra-orbital, 335;  intercostal,  339; 
internal  carotid,  338;  liga- 
mentum  arteriosum,  330;  liga- 
mentum  botalli,  330,  331;  lingu- 
alis,  3355  lumbar,  339;  man- 
dibularis,  335,  338;  maxillaris, 
335;  mesenteric,  339,  340;  poste- 
rior aorta,  320;  pulmocuta- 
neous,  330;  pulmonary,  330;  354; 
sacralis  media,  339,  340;  seg- 
mental,  335;  stapedialis,  159, 
337,  338;  subclaviae  secundarise, 
331;  subclavian,  322,  326,  332, 
339;  supra-orbital,  335;  umbili- 
cal, 322;  vertebralis  cerebralis.^ 
335- 


articulare,   159,  495 

articulates,    258,    259,    463 

articulation  of  jaw,  450 

Artiodactyla,  39 

arytaenoids,   160,   313 

Ascalabotae,    487 

Ascaris,  54 

assimilation,   2 

Ateles,  254 

atlas,  133 

atriopore,  of  Amphioxus,  526  » 

atrium,  324,  353,  357 

auditory    hairs,   488 

auditory   ossicles,    159 

auditory  tube,  494 

auricula,   496,   497 

Auricularia,  536,  537 

axial  muscles,   190,   192,  200,  217 

axial  skeleton,  122 

axillary    arch,    251 

axis,  133 

axolotl,  440 

B 

babyroussa,   274 

Balanoglossus,  26,  303,  533,  534, 
535,  536,  537 

basihyal,   160,  284 

basioccipital,    150 

basipterygium,     169,     175 

basisphenoid,    150 

bat,   183,   184,  292,  394,  433,  496 

bats,    insectivorous,   301 

beak: — of  birds,  84,  105;  of  tur- 
tles, 84. 

beaker   cells,    no 

belly,  of  a  muscle,   197 

biogenesis,   law   of,   15,   58 

Bipinnaria,   536,  537 

birds,  in,  172,  174,  182,  183, 184, 201, 
202,  203,  220,  249,  253,  263,  268, 
272,  290,  296,  315,  352,  353,  354, 
355,  356,  360,  375,  377,  3§5,  386, 
398,  414,  415,  416,  419,  426,  437, 
439,  465,  475,  483,  488,  489,  491, 
495,  500,  504 

birds,   toothed,  272 

bisexual,  49 

bladder,  377,  378 

blastocoele,  59 

blastodermic  vesicle,  73 

blastula,  58 


INDEX 


553 


blood,  318,  356 

blood  vessels,  65,  79,  318 

body  axis,  60,  61 

body,  of  a  vertebra,  127 

body   wall,   365 

bone  complexes  of  skull,   151 

bone,  or  bones  (see  skeletal  ele- 
ments) 

bony  labyrinth,  492,  493 

Bovidae,  276 

Bowman's   capsule,   373,   374 

brachial  plexus,  438,  439,  440,  441 

brain,  communication  with  out- 
side world,  409 

brain,    development    of,    410,    427 

branchial  arches,  155 

branchial   system,  260 

Branchiostoma    (see   Amphioxus) 

broad  ligament,  of  uterus,  387,  390 

bronchi,  origin  of,  270 

bronchioli,   315 

.Bubo,  489 

buccal   glands,  285 

bulbo-urethral   glands,   403 

bull-heads,   475 

bursa   inguinalis,   395 

bursa   ovarica,   387 


caducibranchiate   amphibians,   307 
caenogenetic   characters,    16 
canalis   centralis,   62 
canalis    centralis,    of    spinal    cord, 

406 

canals   of   Lorenzini,   469,  473,   502 
Canidae,   286 
canines,   275 
Canis,  87 
capillaries,  318 
carapace : — armadillo,     85 ;     turtles, 

105 

Carchesium,  8 

cardiac  end  of  stomach,  291 

Carnivora,    37,    38,    175,    239,    250, 

291,  394,  402,  417,  480,  484 
carotid  gland,  288 
carpus  : — nomenclature      of,       I77> 

178,   179,   180;   various  forms  of, 

179;  primitive  condition  of,   179; 

supernumerary    elements,    181. 
cartilage,  or  cartilages  (see  skeletal 

elements) 


cartilage   bones,    148 

cartilage  lateralis,   160 

cassowary,  440 

cat,  271,  276,  287,  419,  491 

Catarrhini,   41,   278 

cauda   equina,  431 

caudal  fin,  164 

caudal  vertebrae,   130 

cavernous   tissue,   398,   399,  404 

cell    membrane,    3 

centers   of  ossification,  83,   148 

centrifugal   nerves,    435 

centripetal    nerves,    408,    435 

centrum    of    a    vertebra,    127,    508, 

5io 

Cephalophus,  112 
cephalopod   eye,   499 
cephalopods,   499,   501 
cerato-hyal,  160,  284 
Cercopithecidae,  45 
cerebellum,    412,    417,    425,    426 
cerebral  hemispheres,  411,  413,  416 
cerebrum,  411,  415-418,  425 
cervical    fistula,    269 
cervical   intumescence,  in  cord,  433 
cervical   rib,   138 
cervical    vertebrae,    130 
Cervus,    299 
Cestracion,    186 
Cetacea,    20,    38,    40,    98,    113,    116, 

175,   184,  275,  277,  285,  300,  316, 

393 

Chelonia,    33,    44 
Chimaera,   489 
chimpanzee,  45 
chiridium,  derivation  from  fin,  186, 

220 

Chironomus,    429 
Chiroptera,   37 
chiropterygium,    167,    184 
chiropterygium,    typical    form    of, 

177 
chiropterygium     vs.      ichthyoptery- 

gium,  184-188 
Chlamydoselachus,  186 
choanae,    269,    478 
Choloepus,    112,    113 
chondrocranium,   145,   146,   153,   156 
chordo  gubernaculi,  395 
Chordata,    538 

chorioid   plexuses,    413,   419,   420 
chorion,    70,    71,   72 


554 


INDEX 


chorionic  villi,   70,   71 
chorioid  coat  of  eye,  498,  501 
chorioid  fissure  of  eye,   500 
chromatin,    54,    58 
chromosomes    54,    58 
chromosomes: — number  of,  54,   55, 

57 

Chrysochloris,    1 12,    113 
ciliary    ganglion,    447 
ciliary   glands,   505 
circulation,    in    selachians,    324,   325 
circumvallate   papillae,   476 
cisternae  chyli,  362 
clavicle,    173,   174,   175 
claws,    105,    106,    107 
cleithrum,    173 
clitoris,   398,  404 
cloaca,  266,  296,  370,  375,  378,  381, 

383,  403 

cloacal    cceca,   266 
cloacal   glands,    no 
closed  type  of  circulation,  317,  318 
Cobitis,    303 
coccyx,    135 
cochlea,   488,   491,  492 
cceca,   of  intestine,  296,   297,  298 
Coelenterata,  59,  60 
Ccelogenys,  86,  491 
ccelom,   64,   66,   258,   365,   368,   369, 

37i,  373,  374,  375,  376,  378,  381, 

382 

colic  cceca,  266 
colon,   298 

colon    labyrinths,    298 
columella   auris,  494 
columns,  of  spinal  cord,  434 
commissures,  of  brain,  418,  427 
concha,  of  ear,  161 
conchae,    of   nose,   480-482 
concrescence   theory,   280 
Condylarthra,   38,  40 
conjunctiva,  498,  504 
contact   sense,  468 
continuity  of  germ  plasm,  58 
continuity    of   life,    12 
conjugation,  6,  49,   50,  51 
conus   arteriosus,   324,   353 
conus    inguinalis,    395 
Cooper's    fascia,    396 
convolutions,  of  brain,  417 
copulation,  49,  380,  381,  398 


coracoid,   174 

coracoid   process,    175 

coral  polyps,  60 

corium,   76,   78 

cornea,    501 

corpora    bigemina,    425 

corpora   cavernosa,   399,   404 

corpora   striata,  413,  416,  426 

corpora  quadrigemina,  425 

corpus    callosum,    418 

corpus  cavernosum  urethrse,  404 

corpus    fibrosum,    398 

corpus   spongiosum,   404 

cortex  cerebri,  417 

costal  cartilages,    136 

cotyledonal    placenta,    72 

cranial  nerves,  427,  442-458 

cremaster,   396 

Creodonta,    37,    40 

cricoid,  314 

cristae   acusticae,   490 

Crocidura,  92 

crocodile,  164,  272,  360,  377,  398, 
399,  483,  491,  495 

crop,   290 

crura  cerebri,  426,  427 

crustaceans,    259,    463 

Cryptobranchus,    31,    235,    305 

crystalline    lens,   423,   498,   501,   502 

ctenoid   scales,   83 

cuticula    of    invertebrates,    76 

cutis,  76 

Cuvier,  theories  of  types,  506,  507 

cyanosis,  357 

cycloid    scales,   83 

Cyclops,  55 

cyclostoma,    266 

cyclostomes,  28,  29,  44,  49,  200, 
267,  286,  287,  289,  305,  347,  365, 
369,  379,  38o,  411,  420,  433,  437, 
444,  461,  487,  488,  489,  490,  516 

D 

Darwin,    Charles : — theory    of    de- 
scent,   510,    512 
Darwin's  point,  on  ear,  497 
Dasyurus,    86 
decidua,  72 
deciduate  placenta,  73 
deer,   299 
Delphinus,   277 


INDEX 


555 


dental    formulae,  276-277 

dermal  bones,  79,  82,   146,   147,   156 

dermal   scutes,  79,  83,   147 

dentary,    156 

dentine,  79 

dermal   canal    system,   448 

descensus  ovariorum,  392 

descenscs    testiculorum,    203,    392, 

393 

Desmodus,  292 
Desmognathns,   309,  343 
developmental    history,    15 
development    of    hair,    96 
devil-fish,   499 
diaphragm,    316 
diapophyses,    138,  508,  510 
Didelphia,    36 

diencephalon,   411,   418-425,   443 
diffuse    placenta,    72 
digestive   cavity,  258,  315 
digestive  glands,  262 
digits: — names  of,  177;  number  of, 

182 ;    reduction    of,    182. 
dinosaurs,  32 
diphyodont    dentition,   280 
dipnoans,    30,    176,    449,    453,    454, 

457,   478 

discoidal  placenta,  72 
dissepiments,    367 
diverticula  of  intestine,  266 
dog,    271,    286 
dog-fish,    414,    415,    419,    446,    461, 

478 

dorsal   fin,    164 
dorsal  nerves,  436 
dragon-fly,    428 
drum  of  ear,  493-496 
Dryopithecus,    44 
duck,    296 

duck-billed   platypus,   33 
duct   of   Cuvier,  322 
ductns   arteriosus,  331 
ductus   Botalli,  331 
ductus    Cuvieri,   322 
ductus  deferens,  377,  383,  385,  386, 

392,    393,    394,    401 
ductus   endolymphaticus,   486 
ductus   venosus   Arantii,   348 
duodenum,   266,   294 
duplex  placenta,  72 
Duplicidentata,   37 


E 


ear,   485-497 

ear,   of  anthropoid,  496-497 

ear,  external,  161,  496,  497 

earth-worm,    460 

ecdyses,  77 ;  of  epitrichium,  95 

Echidna,   mammary  pocket   of,   114 

Echidna,  33,  288 

echinoderms,  26,  258,  536-538 

ectoderm,  60,  63,  257 

ectoturbinalia,  480 

Edentata,   37,   291 

eel,   432 

efferent  nerves,  435 

eggs,  50,  51,  52,  53,  56,  57,  69,  73 

elephant,   274,   484 

enamel,   79 

endochondral   ossification,    148 

endoderm,  60,  63,  257 

endolymph,  488,  492 

endolymphatic    space,    357 

endolymphatic   cavity,   493 

endolymphatic  duct,  486 

endoskeleton,    122 

endoskeleton,  parts   of,    122 

endostyle,   289,   528,   531 

endoturbinalia,  480 

Enteropneusta,   260,    535,    538 

entocone,    279 

epi-hyal,    160,    284 

epidermic  warts,  88,  89,  90 

epidermis,    76,    408 

epididymis,  385,  386,  394 

epiglottis,   160,  313 

epimere,    64,   65 

epimeres,    190,    245 

epimeric  muscles,    190,    192 

epi-otics,   148 

epiphysis,   420,   421,   422 

episternum,    141 

epithelial  bodies,  363 

epithelial  corpuscles,  286 

epitrichium,    95 

eponychium,    95 

epoophoron,    391,    394 

Equus,   390 

erectile   tissue,   398,   399 

Erinaceus,   402,   491 

erythrocytes,   318,   364 

ethmoid,    148 

ethmo-turbinal,    479,    480 


556 


INDEX 


Eurypterus,   524 

Eustachian  tube,  269,  494 

Eutheria,   35 

excretory  tubules,  365,  366 

exoccipitals,    148 

exoskeleton,  79 

exoskeleton   of   reptiles    and    birds, 

84 

external  ear,  161,  496,  497 
external    genitals  : — male,    398-401 ; 
female,     403 ;     development     of, 

403-405. 
exuviae,  77 
external  nose,  484 
eye,   497-505 
eyeball,  muscles   of,  503 
eyeball,    skeletal    elements    of,    144, 

145,   151 

eyeball,   size  of,   503 
eyebrows,   505 
eye-capsules,   145 
eye,  development  of,  422,  423 
eyelids,  498,  503,  504 
eye   of  cephalopod,   499 


Fallopian  tube,  387 

fascia  cremasterica,   396 

femoral  glands,  of  lizard,   no,   III 

fenestra   cochleae,   494 

fenestra  ovalis,  of  ear,  493 

fertilization,  50,  379,  397,  398 

fetal  circulation,  320-324,  332-339, 
342-352,  357 

field-mouse,  91 

fifth  ventricle,  418 

filum  terminale,   431 

fins   of   fishes,    164,    167 

fin,  development  of,  168,  218-219 

fin-fold,  327 

fin-fold  theory,   163,  238 

fin  rays  vs.  digits,  184,  185 

fin    spines,    162 

fin   vs.    hand,    184-188 

fishes,  29,  30,  no,  148,  163,  164, 
165,  167,  175,  192,  200,  245,  247, 
262,  263,  268,  271,  272,  285,  291, 
296,  304,  306,  307,  310,  311,  324, 
330,  339,  342,  347,  352,  353,  355, 
358,  375,  409,  425,  436,  437,  452, 
460,  465,  468,  469,  471,  484,  485, 
488,  490,  491,  503,  517,  519,  527- 


fission,  4,  5 

fiss'ues,    of   brain,  417 

fissue    of    Rolando,   417 

fissue   of   Sylvins,   417 

flat-worms,    520 

flexures,  in  brain,  414 

fly,    428 

fly,    nervous    system    of,    428 

foliate    papillae,    476 

foramen  epiploicum,  294 

foramen  interventriculare,  411,  416, 
420 

foramen  of  Monro,  411 

foramen   ovale    (of  heart),   357 

foramina  of  Majendie,  413 

forebrain,  411,  443 

fossa  rhomboidalis,  427 

fourth  ventricle,   of  brain,  412 

fovea  centralis,  of  retina,  499 

free-limbs: — development  of,  218, 
219;  early  history  of,  175,  176, 
177 ;  modifications  of,  165,  182, 
183,  184;  nomenclature  of,  178; 
origin  of,  165 ;  serial  homology 
of,  178;  typical  skeleton  of,  176. 

free  nerve  endings,  473 

friction  ridges,  90,  107 ;  formation 
of,  88,  89,  90;  ground  plan  of, 
92;  relation  to  pads,  90,  91. 

friction    ridge   patterns,   92,   93 

frog,  198,  263,  264,  285,  290,  307, 
313,  359,  37i,  397,  420,  426,  428, 
430,  439,  440,  489,  526 

frontal   organ,   420 

fundus  of  stomach,  292 

furcula,    174 


galea   aponeurotica,   255 

Galeopithecus,   37 

gametes,  7,  49,   55 

ganglion,  or  ganglia :  —  buccale, 
456;  ciliary,  447,  452,  464;  gas- 
serian,  448,  456;  geniculare,  454, 
456;  jugulare,  453,  456;  laterale, 
453,  456;  list  of,  in  head,  456; 
mandibulare,  456;  of  articulates, 
428,  429,  463;  of  fly,  428,  429; 
of  myriapod,  428;  otic,  451,  452, 
455,  464;  ophthalmicum  profun- 
dum,  456;  ophthalmicum  super- 
ficiale,  456;  petrosum,  454,  456; 


INDEX 


557 


ganglion,  or  ganglia — (Continued) 
semilunare,  448,  449;  spinal,  436; 
spheno-palatine,  452,  455,  456, 
464;  submaxillary,  452,  464; 
sympathetic,  451,  463. 

ganoids,  29,  44,  148,  149,  170,  171, 
173,  176,  184,  230,  270,  311,  369, 
416,  444 

ganoid    scales,    8l 

Gartner's    duct,    391 

gastrocoele,    59,    258,    365 

gastrula,  59,  61,  62,  257,  365 

gall-bladder,    259 

gecko,   487 

geese,   296 

genital  cleft,  403 

genital    ridge,    403 

genital   tubercle,   403 

germ  cells,  7,  366,  368,  378,  379 

germ  glands,  49,  366,  367,  391,  394 

germ  layers,   derivatives  of,  74,  75 

germ    plasm,    continuity    of,    58 

germinal   epithelium,  378,  379 

gibbon,  45 

Gigantostraca,   524 

gill  arches,  fifth  pair,  312 

gill-flap,   306 

gill,  innervation  of,  453,  454;  of 
Amphioxus,  526,  527 ;  of  Balan- 
oglossus,  533,  534;  of  tunicates, 
530,  531- 

gills,   302-307 

gill-slits,  267,  268,  303,  305 

gill  system,  260 

gland,  or  glands: — acinous,  97, 
112;  anal  sacs,  113;  beaker-cells, 
no,  262;  buccal,  285;  bulbo- 
urethral,  403;  carotid,  288;  cil- 
iary, 505;  cloacal,  no;  femoral, 
of  lizard,  no,  in;  germ,  49; 
glandulse  ductus  deferentis,  401 ; 
glandulae  vesicales,  401 ;  harde- 
rian,  504;  integumental,  78,  107; 
intermaxillary,  285;  labial,  285; 
lacrimal,  504;  lingual,  285;  mam- 
mary, 33,  114-118;  meibomian, 
i*3»  5O5;  mesenteric,  362;  molar, 
286;  musk,  110;  necrobiotic,  no; 
odoriferous,  114;  of  mouth  cav- 
ity, 285,  286;  of  mucosa,  262; 
orbital,  286;  parotid,  286;  pre- 
putial,  113,  403;  prostate,  403; 


gland,  or  glands—  (Continued) 
rectal,  114;  retrolingual,  286; 
salivary,  286;  sebaceous,  113; 
•  sublingual,  285 ;  submandibular, 
285 ;  submaxillary,  285 ;  sweat, 
112,  113;  tarsal,  113,  505;  thy- 
mus,  286,  287,  290,  363 ;  thyreoid, 
'286,  289,  290,  363;  types  of,  108, 
109;  Tyson's,  113;  tubular,  97, 
109,  112,  262;  urethral,  401; 
uropygeal,  ill;  vitally  secretory, 
no. 

glandular   area,    114 

glans  penis,  404 

Globicephalus,    183 

glomeruli,  371,  374 

glottis,  270,  310,  311,  312 

Glyptodon,    37 

gnathostoma,  267 

gnathostomes,    29 

Goethe,  theories  of  the  "  Urbild ", 
506,   507 

gonads,   49,   63,   366 

Gorilla,  45 

gorilla  rib,  138 

Grandry's   corpuscles,   474 

grasshopper,    428 

gray  matter,  408,  428 

growth,  2,  3 

greater  curvature  of  stomach,   291 

gubernaculum,   395 

Gymnophiona,  31,  84,  313,  373,  398, 
442,  483 

gynaecomastism,    118 

gyri,    of    hemispheres,    417 


H 


haemal  arches,    126,  508 

haemal    spine,    508,    510 

haemal    system,    318 

Haemamoeba,   5 

haemapophyses,   508,   510 

hair,  469,  471,  472;  arrangement  of, 
86,  87;  development  of,  96;  di- 
rection of,  101,  103,  104,  105; 
distribution  of,  99;  of  mammals, 
95-105;  structure  of,  97,  ioo; 
varieties  of,  97,  98. 

hair  currents,   101 

hair   groups,  87,   88 


558 


INDEX 


hair  of  Man: — direction  of,  102, 
103,  104;  racial  differences,  100; 
shape  of  cross  section,  100. 

harderian  glands,  504 

hard  palate,  270,  271,  479 

Harrimania,  535 

Hatteria,    32 

head   cavities,   461 

head,    formation    of,    124 

head,  relation  to  vertebral  column, 
134 

heart  318,  320,  324,  352-3575  of 
amphibians,  354,  355,  356;  of 
Amphioxus,  353,  528;  of  fishes, 
353,  354;  of  mammals,  356,  357; 
of  reptiles,  356. 

hedgehog,   491 

Heptanchus,    305 

heredity,  material  basis  for,  53,  57, 
58 

hermaphrodites,   379 

hermaphroditic,  49 

hemispheres,  of  cerebrum,  411,  413 

heterodont    dentition,    275 

Hexanchus,   305 

hibernation  of  Dipnoi,  478 

hind  brain,  411 

hip-girdle,   128 

hippocampus,    417 

Hippopotamus,    274,    292,    505 

Holocephali,  444 

Homo,  87,  88 

homodont  dentition,  275 

homology,  of  body  somites,  439; 
of  limbs,  177-180,  237-245;  sex- 
ual, 394,  404,  405 

Homo   neanderthalensis,   42 

Homo    primigenius,    42,    43,    45 

Homo   sapiens,  42,  45 

honeycomb    stomach,    292 

hoofs,  106,  107 

horn,    78 

horns  of  mammals,   106 

horny   structures,    105,   106 

horse,    182,   250,  391 

horse-shoe   crab,   522 

human  phylogenesis,  44,  45 

hydatid   of   Morgagni,  392 

Hydra,  60 

Hydromys,    277 
hyobranchial    apparatus,    283 
hyobranchial    complex,    160,   283 


hyoid  apparatus,    160,  283 

hyoid   arch,    155,    157 

hyoid      bone,      314;      body,      160; 

cornua,     separate     elements     of, 

160,   161 

hyomandibular,    155,    157 
hyperdactylism,    182 
hypermastism,    117 
Hyperoodon,   277 
hyperphalangy,    183 
hyperthelism,    117 
hypertrichosis,   99 
hypobranchial  groove,  289 
hypospadias,   399 
hypothenar   pads,   91 
hypocone,  279 
hypoglossus,  427 
hypomeres,   64,   65,    190,   245 
hypomeric   muscles,    192 
hypophysis,    414,    423,    424,    477 
Hyracoidea,    38 
Hyrax,  38 


Ichthyopsida,    473 
Ichthyopterygium,   167,   184 
Ichthyosaurus,    20,     183,    184,    185, 

186 

ilium,    128,    171 
immortality    of    Protozoa,    5 
incisors,    275 
incus,    159,    150,    195 
inferior  turbinated  bone  (see  max- 

illo-turbinal) 
infundibulum,    424 
ingluvies,  290 
inguinal    ligament,    386,    387,    391, 

395 

inner  ear,  469,  472,  473,  485-493 
insect,    insects,    259,   428,    460,   463, 

506,   512 
Insectivora,     insectivores,     37,     40, 

44,    45,    H3,    175,    239,    250,    277, 

291,  393,  394,  401,  495,  500 
insertion,  of  a  muscle,   195,   197 
integument :— of      amphibians,      83, 

84;   of  Amphioxus,  76;  of  fishes, 

80-83  5    of    invertebrates,    76 ;    of 

mammals,  85;  of  reptiles,  84,  85; 

pigmentation   of,    118-121. 
integumental    glands,    78,    107 
integumental   muscles,    190,    192 


INDEX 


559 


integumental    respiration,    308 
integumental    sense-organs,    448 
interclavicle,    141,    174 
interdigital  pads,  91 
interdural    space,    357 
intermaxillary    glands,    285 
intermuscular   lymph    spaces,   358 
interventricular   foramina,   416,   420 
intestinal    diverticula,    266 
intestinal    respiration,    303 
intestine,  266,  294 
intestine,    length    of,   299,   300,   301 
intestinum    crassum,    294 
intestinum    tenue,    294 
introitus    vaginae,    404 
intromittent  organ,  398 
intumescentiae   of   spinal   cord,  432, 

434 

involuntary   muscles,    189 
iris,   501 
ischium,   172 
iter  a  tertio  ad  quartum  ventricu- 

lum,   412 


Jacobson's   cartilage,   483 
Jacobson's    nerve,    451 
Jacobson's    organ,    483 
jaws,  origin  of,    153,   154 
jumping    mice,   298 


karyokinesis,   55 

keratin,  84 

kidneys,   323,   345,    369,   385, 

kiwi-kiwi,    165 

Krause's  corpuscles,  474 


labial  cartilages,  153,   155 

labial    glands,    285 

labia  minora,  404 

labia    majora,   404 

labyrinth,  452,  485-493;  bony,  492; 
development  of,  485-490;  sen- 
sory areas  of,  488-490. 

Lacerta,    287,   344 

Lacertilia,   32 

lacrimal    apparatus,    504,    505 

lacrimal   apparatus,    504,   505 

lacrimal   bone,   origin   of,   83 


lacrimal  fluid,  504 

lacrimal    glands,    504 

lacunae,    318 

lacunar  circulation,  317 

Laemargus,    470 

lagena,  488,  491,  492 

language,    development    of,    256 

lanugo,    98 

large    intestine,   294 

larynx,  160,  270,  307,  310,  311,  312; 

of     amphibians,     312,     313;      of 

mammals,   314,   315;    of   Saurop- 

sida,   313,   314 
larynx    dorsalis,    310,   311 
larynx  ventralis,  310,  311 
lateral   cartilages,  of  larynx,   160 
lateral    cerebral   tissue,   417 
lateral  line,  192,  448 
lateral    line    organs,   469 
lateral    ventricles,     of    brain,    411, 

416 

lemmings,   298 
Lemuroidea,   41 
Lemurs,    38,   45,   89,    90,    284,    298, 

3H,  394 

lesser    curvature,    of    stomach,    291 

lesser    omentum,    295 

lesser    peritoneal    cavity,    294 

leucocytes,   318,   362 

life  cycle,  9,   10 

ligamentum  : — arteriosum,  330,  331 ; 
Botalli,  330,  331 ;  hepato-gastri- 
cum,  295;  hepato-umbilicale,  347; 
inguinale,  386,  387,  391;  nuchae, 
134;  suspensorium  hepatis,  295; 
teres  hepatis,  347 

limb   muscles,    130 

limb  girdles,   128 

limb   plexuses,   438-442 

limbs,  164-188;  of  primates,  238, 
239 ;  reduction  of,  165,  166 ;  re- 
dundancy of,  166;  serial  homol- 
ogy  of,  177-180,  237-245 

Limulus,    522,   523,   524 

linea  alba,  65 

Lingula,    19 

lion,   282 

Lithobius,   429 

Litopterna,    38 

liver,    266,   294,    323,    348 

lizards,  no,  in,  166,  287,  288,  342, 
344,  398,  416,  420,  422,  483 


560 


INDEX 


lobes,  of  lungs,  315,  316 

lobi  optici,  425,  426 

Loncheres,    86 

love   antics    of   salamanders,    397 

lumbar    intumescence,   433     . 

lumbar    vertebrae,    130 

lumbo-sacral   plexus,   438 

lungless    salamanders,   308 

lungs,    307,    310-316 

lungs,    origin    of,   270 

lung  system,  260 

lymph,   319 

lymph    glands,    318,    362 

lymphatic    nodes,    362 

lymphatics,  65,  318,  358 

lymphatic   spaces,  318,   357,   358 

lymphatic  system,  318,  319,  357- 
364;  development  of,  360;  ori- 
gin of,  363 

lymphatic  vessels,  318,  358 

lymphocytes,  362 

lymph  hearts,  318,  358,  359 

lymphoid    tissue,    362 

lyssa,  385 

M 

Macacus,  91,  92,  93 
macrogametes,    7,   48,    49,    50,   58 
maculae   acusticae,   490 
macula   lutea,   500 
malleus,    159,   45O,   495 
Malpighian   corpuscles,  373 
mammae,    116,    117;    inguinal,    116; 

pectoral,    117;    unusual    position 

of,    117. 
Mammalia,  mammals,  39,   in,   113, 

148,   172,   174,   175,   179,   198,  200, 

201,  203,  206,  212,  215,    2l6,    226, 

227,  229,  236,  247,  248,    249,    250, 

263,  264,  268,  269,  271,    275,    280, 

283,  284,  288,  290,  291,    294,    296, 

301,  314,  315,  319,  322,    324,    331, 

333,  337,  347,  349,  350,  351,  353, 

354,  355,  356,  357,  361,  363,  373, 

377,  385,  386,  388,  389,  390,  391, 

395,  398,  399,  400,  401,  402,  409, 

414,  415,  419,  425,  426,  431,  437, 

439,  444,  450,  451,  452,  455,  465, 

469,  471,  472,  475,  479,  481,  486, 
488,  489,  491,  496,  504,  509 
mammary  glands,  33,   114-118 
mammary    pockets,    114,    115 


mammary    ridge,    116 

mammuth,  42 

mandibular    articulation,    159 

mandibular  cartilages,  153,   156,  157 

Manidae,  85 

marrow,   363 

marsupials,    33,    44,    203,    223,    276, 

284,  292,  297,  357,  388,  389,  399, 

400,  401,  402,  417 
marsupium,   34,    116 
maxillo-turbinal,  479,  480,  481 
Meckel's    cartilage,    156,    159 
median   fins,    163,    164 
medulla,  of  brain,  427 
Medusa,   60 
Megalonyx,    37 
Megatherium,  37 
meibomian    glands,    113,    505 
Meissner's   corpuscles,   473 
membraneous    labyrinth,    485-492 
Merostomata,   522,   523 
mesencephalon,   411,   425,   426 
mesenchyme,  60,  64 
mesenteries,  63,   261,  265,   367,   368 
mesenteric    glands,    362 
mesodseum,   259 
mesoderm,  60,  63,  66 
mesodermic    diverticula,    63-67 
mesodermic  somites,  68,  69 
Mesodonta,   38,  40,   41,  45 
mesomeres,  64,  65 
meso-hypomeres,    64,    65,    68 
mesogastrium,    293 
mesonephridia,  373,  374 
mesonephros,     370,     373-375 J     382, 

385,  386,  390,  391,  394 
mesonephros;   remains  in  Amniota, 

385 
mesonephrotic   duct,   374,   375,   376, 

381,  383,  385,  387 
mesonephrotic    ligament,    386,    390, 

391 
mesonephrotic      system,      373-375 ; 

vestiges   of  in   female,  391 ;   ves- 
tiges of  in  male,  392 
mesonephrotic    tubules,    373,    374 
mesopterygium,    176,    185-187 
mesorchium,   369-387 
mesovarium,    369,    387,    391 
metacoele,  60,  63,  64,  365,  367,  368, 

38i 
metacone,   278 


INDEX 


metaccelic   sacs,   367,   368 

metamerism  in  vertebrate  head, 
460 

metanephridia,  376 

metanephros,  370,  375-377,  3$3 

metanephrotic    system,    375,   377 

metapleural    folds,    527 

metapterygium,    176,    185-187 

Metazoa,   257,   317 

metencephalon,  411,  426,  443 

microgametes,    7,    48,    49,    50,    58 

Midas,  86,  88 

midbrain,    411 

middle   ear,  493-496 

milk  glands,  114-118 

mitosis,   54,   55 

molar   glands,  286 

molars,  275 

mole,  277,  287,  284 

mole,    star-nosed,   90 

monkeys,  90,  91,  92,  112,  223,  254, 
277 

Monodelphia,  36 

Monodon,   277 

monophyodont    dentition,    280 

monorrhine   condition,  476 

monotremes,  33,  44,  236,  255,  337, 
357,  388,  393,  399,  400,  402,  491 

motor  nerves,  434 

motor  roots,  of  nerves,  435,  436 

mouse,  316,  402 

mouth,  466,  467 

mouth   cavity,  466 

mouth,  origin  of,  258,  259 

mucosa,  260,  262 

Miiller's  duct,  382,  387,  388,  391, 
392,  394 

Multituberculata,    35 

Mus,  402 

muscle,  or  muscles: — abductor  cau- 
dae dorsalis,  212 ;  abductor  cau- 
dae ventralis,  212;  abductor  coccy- 
gis,  214;  abductores  breves,  234; 
abductors,  of  pollex  and  minimus, 
236;  achselbogen,  251;  acromio- 
deltoid,  226;  adductores  (fem- 
oris),  229;  adductor  laryngis, 
313;  adductor  mandibulae,  247; 
adductor  mandibulae,  495 ',  an- 
coneus,  222,  226;  auricularis 
anterior,  255;  auricularis  pos- 
terior, 255;  auricularis  superior, 


muscle — (Continued} 

255;  auriculo-labialis,  55;  au- 
riculo-occipitalis,  255 ;  appen- 
dicular,  190,  192,  193,  217-245; 
axial,  190,  192,  200-217;  axil- 
lary arch,  251 ;  biventer  cervicis, 
226,  227;  biceps  femoris,  229; 
brachialis,  226,  227 ;  brachiora- 
dialis,  240;  buccinator,  255;  cau- 
dal, 212-214;  caninus,  255;  cervi- 
calis  ascendens,  208;  coccygeus, 
214;  coraco-brachialis ;  226;  co- 
raco-brachialis  brevis,  222 ;  coraco- 
brachialis  longus,  222;  complexus, 
210;  crureus,  229;  cremaster, 
396 ;  cucullaris,  223 ;  curvatores 
coccygis,  214;  deltoid,  195,  226; 
depressors  of  visceral  arches, 
245,  246,  247;  diaphragm,  316; 
diaphragma  pelvis,  214;  di- 
gastricus,  196,  247,  248,  438; 
dilatator  laryngis,  313;  dorsalis 
antebrachii,  231,  235;  dorsalis 
scapulae,  222 ;  dorso-laryngeus, 
247,  248;  extensor  caudae  later- 
alis,  212;  extensor  caudae  medi- 
alis,  212;  extensor  carpi  radialis, 
237,  239;  extensor  carpi  ulnaris, 
237,  239;  extensor  communis  dig- 
itorum,  235;  extensores  breves, 
231 ;  extensor  radialis,  231 ;  ex- 
tensor ulnaris,  231 ;  facial  mus- 
cles, 254-256 ;  f  emero-fibularis, 
228;  flexores  breves  profundi, 
234;  flexores  breves  superficiales, 
234;  flexor  caudae,  212;  flexor 
carpi  ulnaris,  237,  239;  flexor 
carpi  radialis,  237,  239;  flexor 
digitorum  profundus,  236;  flexor 
digitorum  sublimis,  236;  flexor 
pollicis  longus,  236;  flexor  u'l- 
naris,  234;  flexor  radialis,  234; 
galea  aponeurotica,  55;  gastro- 
chnemius,  239 ;  genio-glossus,  248 ; 
glutaeo-cruralis,  229;  glutaei,  229; 
gracilis,  229;  hyo-glossus,  248; 
hyo-laryngeus,  247;  homology  of, 
194-199;  humero-antebrachialis, 
222,  227;  ilio-coccygeus,  214; 
ilio-costalis,  208;  ilio-femoralis, 
228,  229;  ilio-fibularis,  228,  229; 
ilio-extensorius,  228,  229;  ilio- 


562 


INDEX 


muscle — (Continued} 

psoas,  229;  in  fishes,  200,  201; 
infra-spinatus,  226;  insertion, 
195;  integumental,  190,  192,  249- 
256 ;  intercostales,  externi  et  in- 
terni,  204 ;  intermetacarpales, 
234 ;  intermandibularis  anterior, 
247;  intermandibularis  posterior, 
247;  interossei,  236;  interossei 
dorsales,  236;  interossei  pal- 
mares,  236;  interspinales,  210 ; 
intertransversarii,  210,  211;  inter- 
transversarii  caudae,  213;  inter- 
vertebral  system,  210-212;  invol- 
untary, 189 ;  ischio-cavernosus, 
214,  215;  laryngeus  dorsalis,  247; 
laryngeus  ventralis,  247;  laryngei, 
247,  248,  313;  latissimus  dorsi, 
199,  206,  221,  223,  226,  249,  250, 
251;  levator  ani,  214;  levator  an- 
guli  oris,  255 ;  levatores  arcuum, 
247 ;  levatores  costarum,  204 ; 
levator  menti,  255;  levator  of 
visceral  arches,  245,  246,  247 ;  lev- 
ator scapulae,  222,  223 ;  lingualis, 
249;  longissimus,  208,  212,  215; 
longus  colli,  202;  longus  capitis, 
204;  masseter,  248;  mimetic  mus- 
cles, 253-256,  451 ;  multifidus,  210, 
212;  mylo-hyoideus,  248;  nasal 
muscles,  255;  obliqui  capitis, 
198,  215;  obliqui  oculi,  216,  447; 
obliquus  capitis  inferior,  212;  ob- 
liquus  capitis  superior,  212;  ob- 
liquus externus  abdominis,  198, 
203,  204 ;  obliquus  inferior  oculi, 
447;  obliquus  internus  abdominis, 
203,  204;  obliquus  superior  oculi, 
447;  obturator  externus,  229; 
obturator  internus,  229;  occipito- 
frontalis,  255;  of  eyeball,  216, 
447 ;  of  hip  girdle  and  thigh,  227, 
230;  of  shoulder  and  upper  arm, 
217,  227;  of  the  free  limbs,  230, 
237;  opponens  hallucis,  242;  op- 
ponentes  of  pollex  and  minimus, 
236;  orbicularis  oris,  255;  orbicu- 
laris  oculi  (palpebrarum),  255; 
origin,  195 ;  palmaris,  236 ;  pal- 
maris  profundus,  233;  palmaris 
longus,  236;  palmaris  superfici- 
alis,  233 ;  panniculus  carnosus,  250, 


muscle —  (  Continued} 

251;  parietal,  190,  192;  patagial 
muscles,  249;  pectoralis  abdomi- 
nalis,  251;  pectoralis,  199,  222, 

226,  249,  250,  251,  253;  pectoralis 
major,     226;     pectoralis     minor, 
226 ;  peronaeus  brevis,  239 ;  pero- 
naeus  longus,  239;  peronaeus  ter- 
tius,    241;    petro-hyoideus,     198; 
pharyngeal  constrictors,  249;  piri- 
formis,  230;   platysma,    196,   253; 
prevertebral,    202;    principles    of 
muscle    formation,     198;     proco- 
raco-humeralis,     222,     226;     pro- 
nator,   234,    237;    pronator   teres, 
240;     pronator     quadratus,    240; 
propatagialis,    249;    pterygoideus 
externus,  248;  pterygoideus  inter- 
nus,   248;    pubo-coccygeus,    214; 
pubo-ischio-femoralis        externus, 

227,  229 ;     pubo-ischio-femoralis 
internus,    228,    229;    pubo-ischio- 
tibialis,     228,     229;     pubo-tibialis, 
228 ;  pyramidalis,  203 ;  quadratus 
labii     inferioris,     255;     quadratus 
lumborum,   203;   quadriceps   fem- 
oris,   229;    recti   oculi,    216,   447; 
rectus      abdominis,       199,      203; 
rectus      capitis      anterior,      204; 
rectus     capitis     posterior    major, 
212;      rectus      capitis      posterior 
minor,  212;    rectus   femoris,  229; 
rectus  externus  oculi,  447;  rectus 
inferior  oculi,  447;    rectus   inter- 
nus   oculi,    447;    rectus    lateralis 
capitis,    204;    rectus    superficialis, 
222;    rectus    superior   oculi,   447; 
retractor  bulbi,  447 ;   rhomboidei, 
206,     226 ;     rhomboideus     capitis, 
226;     rhomboideus     dorsi,     226; 
rhomboideus     major,     206,     226; 
rotatores,  210;  sacro-coccygei  an- 
teriores,       214;        sacro-coccygei, 
posteriores,    214;     sacro-lumbalis, 
207 ;    sacro-transverso-transversa- 
lis   system,   207,   208,   215;    sarto- 
rius,  229;  scalenus  anterior,  204; 
scalenus    medius,    204 ;     scalenus 
posterior,      204;      semi-membra- 
nosus,     229;     semi-spinalis,    210; 
semitendinosus,  229;  serratus  an- 
terior   (magnus),    223;    serratus 


IXDEX 


563 


muscle — (Continued) 

magnus,  222,  223;  serrati  poste- 
riores,  203,  206;  soleus-gastro- 
chnemius,  239;  sphincter  ani, 
214;  sphincter  colli,  253;  sphincter 
cloacae,  250;  sphincter  marsupii, 
250;  spinalis  capitis,  209;  spinalis 
cervicis,  208 ;  spinalis  dorsi,  208 ; 
spino-deltoid,  226 ;  spino-spinalis 
system,  208,  215 ;  spino-transver- 
salis  system,  207;  splenius  capi- 
tis, 207;  splenius  cervicis,  207, 
215;  stapedius,  196,  248,  438,  495; 
sternalis,  253;  sterno-cleido  mas- 
toideus,  223;  sterno-cleido  mas- 
toideus,  457;  striated,  189,  190; 
stylo-glossus,  248;  stylo-hyoideus, 
248;  subclavius,  226;  subcutaneus 
faciei,  255;  supinator,  231,  237; 
supinator  brevis,  240;  supinator 
longus,  240;  supracoracoideus, 
222,  226;  supra-spinatns,  226; 
temporalis,  190,  248;  tenuissimus/ 
229;  tensor  tympani,  248,  495; 
teres  major,  223,  226;  teres  minor, 
226;  tibialis  anterior,  239;  tibialis 
posterior,  239;  trachelor-mastoid, 
208;  transversalis  abdominis,  203, 
204;  transversalis,  colli,  208; 
transverso-spinalis  system,  210, 
215;  transversus  thoracis,  204; 
trapezius,  206,  222,  223,  457;  tri- 
angularis  sterni,  204;  triceps,  222, 
226;  unstriated,  189;  vastus 
group,  229;  visceral,  190,  192,  245- 
249;  voluntary,  189,  190;  zygoma- 
ticus,  255 

muscles  of  Necturus,   191-193,  220- 
222,  227-229,  230-235 

muscles,   of   alimentary   canal,   290, 
291 

muscle  somites,   192 

muscular   homology,    194-199 

musculosa,  260 

musk  glands,  no 

muskrat,  297 

myelencephalon,  412,  426,  42;,  443 

myocommata,  27,   192 

myology,   comparative,    195 

myomeres,  27  (see  also  myotomes) 

Myopotamus,  86 

myotomes,  192,  461 


myotomic  buds,  217,  218,  219 

myriapod,  428,  429,  460 

myriapod,  nervous  system,  428 

Myrmecophaga,  277 

Myrmecophagidae,  33 

Myxine,  437,  476,  488,  489,  516 

myxinoids,  49 

nails,   105,  106,  107 

nares,  anterior,  478 

nares,  posterior,  478 

narwhal,  277 

nasal  capsules,  145 

nasal   cavities,  269 

Nasodon,  281 

naso-lacrimal  duct,  504 

naso-pahtine  canal,  479 

Neanderthal  man,  42 

neck,    132,    133 

neck,   formation  of,   130 

Necturus,  128,  138,  171,  172,  183, 
186,  191,  220,  227,  228,  229,  230, 
231,  232,  233,  234,  235,  237,  238, 
240,  246,  247,  312,  313,  430,  442 

nemerteans,   520,  521 

nemertean  theory  of  vertebrate  an- 
cestry, 520,  521,  522 

nerve,  or  nerves : — abducens,  216, 
443,  445,  446,  447,  461,  462 ;  acces- 
sorius,  443,  452,  455,  457;  acusti- 
cus,  443,  448,  452,  462;  auditorius 
(see  acusticus)  ;  ansa  hypoglossi, 
458;  brachial  plexus,  438,  439- 
441 ;  buccalis,  449 ;  chorda  tym- 
pani, 450,  451 ;  communicans  IX, 
454,  456;  facialis,  196,  246,  248, 
256,  443,  448-452;  453,  454,  461, 
502;  glosso-pharyngeus,  246,  247, 
443,  451,  452,  457,  461,  502;  gus- 
tatorius,  452 ;  hyomandibularis, 
449;  hypoglossus,  443,  457-459, 
462,  463;  intestinalis,  454,  457; 
Jacobson's,  451,  454,  456;  later- 
alis  X,  453,  457;  lingualis,  452; 
454;  lumbo-sacral  plexus,  438; 
mandibularis  externus  VII,  449; 
mandibularis  internus  VII,  449; 
mandibularis  V,  449,  451 ;  maxil- 
laris,  449;  motor,  434;  motor 
oculi,  216,  443,  445,  446,  447,  462; 
occipital,  <\/\/\ ;  occipito-spinal, 
444;  olfactorius,  443,  444;  oph- 
thalmicus,  451;  ophthalmicus 


564 


INDEX 


profundus,  449,  462,  463;  oph- 
thalmicus  superficialis  V,  44§; 
ophthalmicus  superficialis  VII, 
448;  opticus,  443,  444;  palatinus, 
449;  palatinus  major,  450, 
456;  patheticus  (v.  trochlearis) 
peripheral,  434,  437;  petrosus 
profundus  major,  456;  petrosus 
profundus  minor,  456;  petrosus 
superficialis  major,  450,  456;  pe- 
trosus superficialis  minor,  456; 
plexus,  196,  197;  plexus  brachi- 
alis,  438,  439-441;  plexus  lumbo- 
sacralis,  438;  plexuses,  of  sym- 
pathetic system,  464;  pneumo- 
gastric  (v.  vagus)  ;  posttrematici, 
454,  462,  praetrematici,  454,  462; 
sensory,  434;  spinal  accessory 
(v.  accessorius)  ;  spino-occipi- 
tal,  444;  stapedialis,  451;  sym- 
pathetic plexuses,  464;  sympa- 
thetic system,  451,  463,  464;  ter- 
minalis,  445;  trifacial  (v.  trig- 
eminus)  ;  trigeminus,  247,  248, 
443,  447,  448-452,  461,  495 ;  troch- 
learis, 216,  443,  445,  446,  447, 
449,  462;  tympanic,  451,  454, 
456;  vagus,  247,  443,  452-457, 
461 

nerves,  motor,  434 

nerves,  peripheral,  434,  437 

nerves,  sensory,  434 

nerve  supply  to  muscles,  438 

nervous  system  : — anlage  of,  62 ; 
origin  of,  406,  407 

neosternum,    138 

neostoma,    154,  424 

nephridia,  366,  368,  370,  371,  381, 
382,  383 

nephrostomes,  366,  373,  374,  382, 
383 

neural  arches,  126,  508 

neural  processes,  126 

neural  spine,  508,  510 

neural  tube,  62,  406;  development 
of,  407,  408;  early  history  of, 
406,  407 

neurapophyses,  508,  510 

neurenteric  canal,  258 

neurilemma,  434 

newt,  313 

nictitating  membrane,  504 


nipples,     115-118;     morphology    of, 

115;  rudimentary,   117 
noduli    lymphatici    aggregati,     362, 

363 

Nomarthra,  37 

nose,   cartilages   of  external,    151 

nose,  external,   151,  484 

notochord  (see  under  skeletal  ele- 
ments) 

notochord  : — of  Amphioxus,  123, 
527;  of  Balanoglossus,  535;  of 
tunicates,  531 

nucleus,  3 

O 

obturator   foramen,    172 

occipital   condyles,    133 

odontoblasts,  273 

odontoid  process  of  axis,   133 

oesophagus,  290 

Oken,    Lorenz,   his   theories,    506 

olfactory  buds,  485 

olfactory  lobes,  416,  445 

olfactory  nerve,  416 

olfactory  pit   of  Amphioxus,  445 

olfactory  surface,  increase   of,  479, 

482 

omasus,  292 
omentum,  293 
ontogenesis,   15 
ontogenesis,    laws    of,    20-25 
oogonium,    55,    56,   57 
open  type  of  circulation,  317 
operculum,  of  ear,  493 
operculum,  of  gills,  157,  306 
Ophidia,  32 
opossum,  33,  276 
optic  cup,  422,  423 
optic  lobes,  411,  425 
optic  stalks,  445 
optic  vesicle,  422,  423 
optici  thalami,  425 
opisthocoelous  vertebrae,  131 
orang-utan,   45 
orbital  glands,  286 
organs    of    Corti,    492 
organs   of  Giraldes,  392 
organ   of   hearing,  485-497 
organ  of  Rosenmiiller,  391 
origin,  of  a  muscle,   195 
Ornithorhynchus,  33,  90,  432 
Orthagoriscus,   430,   431 


INDEX 


565 


ossicula  auditus,   159 

ostium  tubae,  382 

ostrich,    165,    172,   433 

otic   capsules,    145 

otic  vesicle,  473,  486 

otter,  505 

ova,  366,  379,  382,  389 

ovaries,  49,  367,  379,  383,  387,  389, 

39i,  393,  394 
oviduct,  382,  383,  385,  387,  388,  389, 

39i,  394 

ovum,  48,  49,  50,  51,  55,  58 
Owen    Sir    Richard,    his    theories, 

507-512 
owl,  489 
ox,   182,  491 


Pacini's   corpuscles,   474 

pads,    mammalian,    90,    91 

Palaeostoma,  154,  424,  478 

palatine   cleft,  271 

palingenetic  characters,  16 

pallium,   416 

palm  print  of  boy,  94 

pancreas,   266,^294 

Pantotheria,  35,  44 

papilla  acustica  basilaris,  490,  491 

papilla  acustica  legenae,  490 

parachordal  elements,   144,   145 

parachordal    region    of    head,    124, 

144,  46o 
paracone,  278 
paradidymis,  392 
Paramoecium,  4,  6 
paraphysis,  420,  422 
parapophyses,    508 
pars  olfactoria,  of  nose,  478 
paraseptal  cartilage,  483 
parasternum,   141 
pars  respiratoria,  of  nose,  478 
parathyreoid  bodies,  289 
paired  fins,  163.  164 
parietal  foramen,  420 
parietal  eye,  422 
parietal   mesoderm,  63,  66 
parietal   muscles,    190,    192 
parietal  organ,  420 
paroophoron,  391,  394 
parotid   glands,  286 
parrots,    296 
patagium,  249 


patella,    178 

paunch,  292 

pearl  organs,  472 

pectoral  fins,  164 

pectoral  girdle,  130 

pedunculi  cerebri,  426 

pelvic  girdle,   169-172;  phylogenesis 

of,  170-172 

pelvis,  of  kidney,  376 
penis,  398,  399,  400,  401 
penis,  position  of,  399,  400 
perennibranchiate    amphibians,    307, 

330 

perilymph,  492 
perilymphatic  cavity,  492-493 
perilymphatic  space,  357 
perinaeum,   389,   401 
perineurium,  434 
Perissodactyla,    39 
peristalsis,   265 

peritoneal  cavity,  367-369,  380 
peritoneal  cavity,  lesser,  294 
peritoneum,   66,  261,   293,   294,  316, 

367,  374 

perivisceral  fluid,  317 
perspiration,    112 
pes   hippocampi,   417 
petrel,  478 

Petromyzon,  289,  420,  477,  488 
Peyer's   patches,   362 
phallus,    398 
pharyngeal  pockets,  268 
pharyngeal  pouches,  265,  266 
pharyngo-oesophageal       respiration, 

308,  309 

pharynx,  265,  266 
Phascoloarctus,   297 
philthrum,  271 
phylogenesis,  15 
phylogenesis,   laws   of,    16-20 
phylogenesis  of  man,  44,  45 
phylogenetic  tree,  of  mammals,  36, 

39;   of  vertebrates,  28 
pig,  182,  274,  361,  391,  437,  489 
pigment,   78,    118-121 
pigment   speck,  of  Amphioxus,  445 
pineal  gland,  421 
pineal  organ,  420 
pinna,  of  ear,  161,  496,  497 
Pinnipedia,   38 
Pisces,  29,  30 
Pithecanthropus,   43 


566 


INDEX 


pituitary  body,   424 

placenta,  34,  70,  71,  72,  73,  377,  388 

placoderms,  29,  83,  524,  525 

placoid  scales,  80,  81,  267 

placoid  scales  on  teeth,  154 

plasma,  318,  319 

plastron   of   turtles,    105 

platyhelminths,  520 

Platyrrhini,  278 

Pleisiosaurus,   185 

pleura,  66,  316 

pleurapophyses,    138,   508,    510 

pleuro-peritoneal   cavity,    64,    66 

plexus,  blood  vessels  in  brain,  413, 

419,  420 

plexus  brachialis,  438,  439-441 
plexuses,  chorioid,  413,  416,  419,  420 
plexuses,  of  nerves,  438 
plexuses,   sympathetic,  464 
plexus  formation,  nerves,  438 
plexus   lumbo-sacralis,  438 
plica  fimbriata,  284 
plica   semilunaris,   504 
pneumatic  cyst,  310,  311 
poikilothermous   animals,   356 
poison  fangs  of  serpents,  280 
polar  globules,  56,  57 
Polypterus,    171,    186,   187,   188,  311 
Polypterus,   ribs   of,    137 
polyspermy,   53 
pons  Varolii,  426,  427 
Pontoporia,  301 

pori  abdominalis,  369,  380,  381 
porpoise,  478,  491,  505 
postbranchial  bodies,  288,  289 
posterior  limb,   169 
posterior  nares,  269,  478 
post-minimus,   182,  242 
praechordal  elements,   144,   145 
praechordal  portion  of  head,  460 
praechordal  region  of  head,  124,  144 
pre-hallux,    182 
prehistoric  man,  42 
premolars,  275 
pre-pollex,  182,  242 
preputial  glands,   113,  403 
Prevertebrata,  26 
primary  body  cavity,  365 
Primates,  38,  40,  41,  90,  91,  92,  93, 

165,   175,  216,  223,  238,  250,  277, 

291,  394,  402,  417 
Primates,  limbs,  239 


primitive  dentition,  277 

primordial    brain,   411 

primordial   skull,   145 

Proboscidea,    38,    39 

Procavia,   38 

processus   vaginalis,   395 

precocious  vertebrae,    131 

procoracoid,    174 

proctodaeum,  259 

pronephridia,  371 

pronephros,  37<>373 

pronephrotic  duct,  370 

pronephrotic  tubules,  371,  374 

Propithecus,    300 

propterygium,    176,    185-187 

prosencephalon,  411 

prostate  gland,  403 

prostatic  vesicle,  393,  394 

Proteus,  231,  247,  300,  312,  313 

Protochordata,  26 

protocoele,  63,  64,  365,  367 

protocone,    278 

protoplasm,    characteristics    of,    I 

protostome,  59,  61 

Prototheria,  35 

Protozoa,  3,  257 ;  immortality  of,  5 ; 

conjugation   of,  6 
pseudohypertrichosis,  99 
Pterichthys,    524 
Pterodactyl,   183,   184 
pterygoid   canal,  450 
pubic  bones,   172 
pubo-ischiadic   symphysis,    172 
pulmonary    system,    260,    270,    304, 

308,  310-316 
pulsating   vessels,   317 
pyloric  coeca,  266 
pyloric  end  of  stomach,  291 
pylorus,    265,    291 

R 

rabbit,  297,  334,  345,  346,  491 

race  history,  15 

Rana,   313,   489,   526 

Ranodon,  186 

rat,  336,  338 

ratio  of  surface  to  mass,  3,  261,  262 

ray,  453 

receptacula  chyli,  362 

recessus  utriculi,  487 

rectum,  298,  389 

red  blood   corpuscles,  318,  364 


INDEX 


567 


reduction   of  limbs,    165,   166 

reduction   of  teeth,  277 

Reissner's   membrane,   492 

renal  corpuscle,  373 

reproduction   by   fission,  4 

reproductive   system,   378-405 

reptiles,  44,  172,  201,  203,  212,  220, 
229,  248,  253,  255,  268,  272,  280, 
296,  328,  330,  331,  352,  353,  355, 
356,  358,  375,  377,  385,  386,  399, 
409,  415,  4i6,  *I9,  425,  426,  465, 
472,  487,  488,  504 

respiration,  301-316;  in  amphibians, 
306-308,  312-313;  in  Amphioxus, 
304;  in  Balanoglossus,  304;  in 
fishes,  304,  305,  306;  integumen- 
tal,  308;  intestinal,  303;  pharyn- 
go-oesophageal,  308 ;  pulmonary, 
307,  310-316 

reticulum,  292 

retina,  409,  422,  497,  498 

retrolingual  glands,  286 

rhinencephalon,   416,   445 

Rhineura,  20 

Rhinoceros,   394 

rhombencephalon,  427 

Rhynchocephalia,    32 

ribs,  126,  135,  136,  137,  138;  distri- 
bution of,  137,  138;  two  types  of, 
135,  137;  variation  in,  129,  130 

rodents,  37,  45,  175,  203,  223,  239, 
274,  277,  297,  298,  316,  394,  401, 
417,  480,  495,  500 

roots,  of  nerves,  435 

roots,  of  teeth,  273,  274 

rods  and  cones,  of  retina,  422,  467, 
498 

rods  of  Corti,  492 

rostral  plates  of  skull,  146 

round  ligament  of  uterus,  387,  390 

rumen,  292 

ruminants,  292,  297,  484 


sacculus,   487,   488 
saccus    endolymphaticus,   486 
sacral  vertebrae,   129,  130 
sacrum,  variation  in,   128,  129 
St.  Hilaire,  his  theories,  506,  512 
salamanders,  179,  185,  186,  192,  220, 

223,  307,  3o8,  309,  354,  383,  388, 

397,  429,  493 


saliva,  285 

salivary  glands,  285 

Sauropsida,  no,  200,  264,  283,  284, 
285,  294,  296,  313,  314,  319,  322, 
323,  324,  347,  36o,  373,  375,  377, 

391,  444,  457,  464,  479,  491,  495, 
497,   499 

scales  : — ctenoid,  83 ;  cycloid,  83 ; 
epidermic,  84;  of  fishes,  80;  of 
ganoids,  81 ;  of  mammals,  85,  86, 
90;  of  palmar  and  plantar  sur- 
faces, 95;  of  Stegocephali,  84; 
placoid,  80;  structure  of,  80 

Scaphyrhynchus,    171 

schizocoele,   68 

sclerotic  coat,  of  eye,  423,  498,  501 

scorpion,   524 

scrotal  raphe,  404 

scrotal  sac,  395,  396 

scrotum,  394,  395 

sculpin,  419 

scutes,  79,  83,   146 

seal,  282,  316 

sebaceous  glands,  113 

segmentation  of  head,  458,  460,  461 

selachians,  29,  44,  152,  153,  170,  172, 
173,  184,  185,  217,  218,  219,  253, 
272  288,  305,  324,  325,  326,  327, 
328,  340  341,  369,  38o,  381,  382, 

392,  416,  419,  420,  436,  460,  486, 
488,  489,  490,  512,  515 

selachians,    circulation   in,   324,    325 

selachians,  skull  of,  145,  146,  153 

sella  turcica,  424 

semicircular   canals,  487 

seminal  fluid   (see  spermatic  fluid) 

seminal  groove,  399 

sensations,  467,  468 

sense  of  contact,  468 

sense-organs,  408,  465-468 

sense-organs,    accessory    parts,    467 

sense-organs,  cells  of,  466 

sense  organs  of  head,  143 

sensory  cells,  408,  466,  467 

sensory  nerves,  408,  434 

sensory  roots  of  nerves,  435,  436 

septum  atriorum,  354 

septum  linguae,  285 

septum  pellucidum,   418 

serial  homology  of  limbs,  178,  237- 

245 

serosa,  260 


568 


INDEX 


serous   cavities,   318 

serpents,    285 

sesamoid  bones,  178 

sex   determination,   403 

sexes,  definition  of,  49 

sexual    homologies,    393,    394,    404, 

405 

sexual  kidney,  382,  383,  385 
sharks,    388 
sheep,  276 
shoulder-girdle,    172-175;    of  Amni- 

ota,     174,     175;     of     amphibians, 

174;  of  ganoids,  173;  of  teleosts, 

174;  of  selachians,  173 
shrew-mouse,  92 
sigmoid   flexure,  298 
Simplicidentata,  37 
sinuses,  in  bones  of  face,  482 
sinus  maxillaris,  482 
sinusoids,  318 

sinus  utriculi  posterior,  487 
sinus  utriculi  superior,  487 
sinus,  venosus,   324,   326,   340,    341, 

353-357 

Siren,  165,  246,  442,  471 

Sirenia,  38,  39,  98,  113,  H7,  3i6, 
393 

skates,    453 

skeletal  elements: — abdominal  ribs, 
141;  accelous  vertebrae,  131;  ali- 
sphenoids,  148;  alveoli  of  jaws, 
273;  amphicoelous  vertebrae,  127; 
angulare,  156;  appendicular  skel- 
eton, 122;  appendicular  skeleton, 
162  ff. ;  archisternum,  138 ;  artic- 
ulare,  159,  495;  arytaenoids,  160, 
313;  atlas,  133;  auditory  ossicles, 
159 ;  auricula,  496,  497 ;  axial  skele- 
ton, 122;  axis,  133;  basihyal,  160, 
284;  basioccipital,  150;  basiptery- 
gium,  169,  175 ;  basisphenoid, 
150;  bone  complexes  of  skull, 
151;  bony  labyrinth,  492;  bran- 
chial arches,  152,  155 ;  carpus,  177- 
181 ;  cartilage  bones,  148 ;  carti- 
lago  lateralis,  160;  centers  of  os- 
sification, 148;  centrum  of  verte- 
bra, 127;  ceratohyal,  160,  284; 
cervical  rib,  138;  cervical  verte- 

•  brae,  130;  chondrocranium,  145, 
146,  153,  156;  clavicle,  173-175 ; 
cleithrum,  173;  coccyx,  135;  col- 


skeletal  elements — (Continue  d^ 
umella  auris,  494;  conchse  of 
nose,  480-482 ;  coracoid,  174 ',  cos- 
tal cartilages,  136;  cricoid,  314; 
dentary,  156;  dentine,  79;  der- 
mal bones,  79,  82,  146,  147,  156; 
dermal  bones  of  skull,  82,  146, 
147,  156;  diapophyses,  138;  dor- 
sal vertebrae  (see  thoracic  verte- 
brae) ;  ear,  external,  496,  497; 
ectoturbinalia,  480;  endochondral 
ossification,  148;  endoturbinalia, 
480 ;  entoglossum,  283-285 ;  epi- 
glottis, 160,  313;  epihyal,  160, 
284;  epiotics,  148;  episternum, 
141 ;  ethmoid,  148 ;  ethmo-turbi- 
nal,  479,  480;  exoccipitals,  148; 
external  ear,  496,  497;  eyeball, 
skeleton,  elements  of,  144,  145, 
151;  eye  capsules,  145;  falci- 
forme,  182;  fangs  of  serpents, 
280;  fin  spines,  162;  free-limb, 
skeleton  of,  176-178;  f  rentals, 
82,  83,  147;  furcula,  174; 
ganoid  scales,  81 ;  gill  arches, 
312-314;  haemal  arches,  126; 
hard  palate,  270,  271,  479;  hip- 
girdle,  128;  hyobranchial  appa- 
ratus, 283 ;  hyobranchial  complex, 
160,  283;  hyoid,  160,  161 ;  hyoid 
apparatus,  160,  283;  hyoid  arch, 
155,  157;  hyoid  bone,  314;  hyoid 
complex,  314;  hyomandibular, 

155,  157;    ilia,    171;    ilium,    128; 
incus,  159,  450,  495;  inferior  tur- 
binated  bone    (see  maxillo-turbi- 
nal)  ;      interclavicle,      141,      174; 
ischia,   172 ;   Jacobson's   cartilage, 
483;    jaws,    origin    of,    153,    154; 
labial   cartilages,    153,    155 ;    laby- 
rinth,   bony,    492;     lacrimal,    83, 
147;   lateral  cartilages  of  larynx, 
160;     limb-girdles,     128;     lumbar 
vertebrae,   130;  malleus,   159,  450, 
495 ;    mandibular    cartilage,    153, 

156,  157;    maxillaries,    83,    156; 
maxillo-turbinal,   479-481 ;    Meek- 
el's    cartilage,     156,     159;    meso- 
pterygium,    176,     185-187;    meta- 
pterygium,     176,     185-187;     nasal 
capsules,    145;    nasals,    82,    147; 
naso-turbinal,     479,      480;      neo- 


INDEX 


569 


skeletal  elements — (Continued} 
sternum,  138;  neural  arches,  126; 
neural  processes,  126;  nose,  car- 
tilages of  external,  157;  noto- 
chord,  26,  27,  63,  122,  123;  noto- 
chord,  of  Amphioxus,  527;  noto- 
chord,  of  Balanoglossus,  535; 
notochord,  of  tunicates,  531 ;  oc- 
cipital condyles,  133 ;  odontoid 
process  of  axis,  133;  omoster- 
num,  142;  operculum,  of  ear, 
493 ;  operculum,  of  gills,  147,  157 ; 
opercular  bones,  147 ;  opisthocoe- 
lous  vertebrae,  131 ;  opisthotics, 
148;  optic  capsules,  145;  orbitals, 
83,  147;  orbitosphenoids,  148;  os 
entoglossum,  283-285;  os  falci- 
forme,  182 ;  ossicula  auditus, 
*59;  ossification,  centers  of,  148; 
otic  capsules,  145;  palate,  270; 
271 ;  palatines,  83,  147,  156 ;  pa- 
tella, 178;  parabasal,  83,  147; 
parachordal  elements,  144,  145; 
paraseptal  cartilage,  483 ;  para- 
sphenoid,  148 ;  parasternum,  141 ; 
parietals,  82,  83,  147;  pectoral 
girdle,  130,  169-172;  petrosal 
bone,  493;  petrosals,  148;  pinna, 
496,  497;  placoid  scales,  80,  81, 
267;  placoid  scales,  as  teeth,  154; 
plastron,  of  turtle,  105;  pleura- 
pophyses,  138;  post-frontals,  82, 
147 ;  post-temporals,  174 ;  prae- 
choidal  elements,  144,  145;  prae- 
f rentals,  82,  147;  praemaxillary, 
156;  praesphenoid,  150;  primor- 
dial skull,  145;  precocious  verte- 
brae, 131;  procoracoid,  174;  pro- 
otics,  148;  propterygium,  176, 
185-187;  pterygoids,  147,  156; 
pubic  bones,  172 ;  quadrate, 
156,  450,  495;  quadrate- jugal, 
496;  ribs,  126,  129,  130,  135- 
138;  ribs,  abdominal,  141;  ribs, 
distribution  of,  137,  138;  ros- 
tral plates,  146;  sacrum,  129; 
sacral  vertebrae,  129,  130; 
scales,  ganoid,  81 ;  scales,  placoid, 
80,  81,  267;  scapula,  174;  scapulo- 
coracoid,  173;  sesamoid  bones, 
178;  shoulder  girdle,  173-175; 
sinuses,  in  bones  of  face,  482; 


skeletal  elements — (Continued") 
skull,  amniote  stage,  150,  151 ; 
skull,  amphibian  stage,  150; 
skull,  bone  complexes  of,  151; 
skull,  development  of,  142-145; 
skull,  ganoid  stage,  146;  skull,  of 
cyclostomes,  143;  skull,  of  sela- 
chians, 144,  145,  146,  153 ;  skull, 
primordial,  145;  skull,  selachian 
stage,  144-146;  spinous  processes, 
134;  spiracular  cartilage,  155; 
squamosals,  82,  83,  147;  stapes, 
159,  337,  495;  sternebrae,  140; 
sternum,  138,  139,  142;  stylo-hyal, 
161,  284;  styloid  process,  161,  284; 
supra-clavicles,  174 ;  supra-clei- 
thra,  174 ;  supra-occipital,  82,  147 ; 
suspensorium,  of  jaws,  155;  tar- 
sal  cartilages,  504;  tarsus,  177- 
182;  teeth,  267,  271-283;  teeth, 
evolution  of  shapes,  278-280; 
teeth  of  birds,  84;  teeth  of  se- 
lachians, 81 ;  teeth,  origin  of,  81, 
I53>  154;  teeth,  replacement  of, 
280-283;  thecae  of  jaws,  273; 
thoracic  vertebrae,  130,  thyreo- 
hyal,  284,  314;  thyreoid,  314; 
thyreoid  cartilage,  160;  tongue- 
bars,  of  Amphioxus,  290;  tooth, 
Structure  of,  80;  trabeculae, 
144;  trachea,  270,  312;  tracheal 
pieces,  160,  313;  tracheal 
rings,  313,  314;  trunk  vertebrae, 
130;  turbinalia,  479-482;  tympanic 
bone,  496;  tympanic  bulla,  496; 
tympano-hyal,  161,  284;  typical 
vertebrae,  507-511;  uncinate  pro- 
cesses, 136;  urostyle,  136;  verte- 
brae, 129,  130;  vertebrae,  develop- 
ment of,  124-127;  vertebrae,  typi- 
cal, 507-511;  vertebral  column, 
130-133;  vertebral  column,  devel- 
opment of,  124-127;  visceral 
arches,  267;  visceral  skeleton, 
122,  152-162;  vomero-nasal  car- 
tilage, 483;  vomers,  83,  147,  150; 
Weber's  apparatus,  486;  wish- 
bone, 141. 

skeleton,  relation  to  soft  parts,  123 

skin,  77 

skin  color  in  Man,  119 

skin,  pigmentation  of,    118-121 


INDEX 


skull: — amniote  stage,  150,  151; 
amphibian  stage,  150;  develop- 
ment of,  142-145;  ganoid  stage, 
146;  selachian  stage,  144-146. 

slime  canals,  469 

sloths,  113,  316,  393 

small   intestine,  294 

smell,  468,  476-485 

smell-buds,  471 

snakes,  110,  166,  377,  398,  432,  433, 
441,  483,  487,  505 

soft  palate,  266,  271 

soma,  7,  11,  48,  58 

somatic  cells,  55 

somites,  27 

somites  of  head,  458-463 

sparrow,  419 

spermatic  cord,  396 

spermatic  fluid,  49,  52,  380,  381,  383 

spermatogonium,  55,  56,  57 

spermatophores,  383 

spermatozoon,  48,  49,  50,  51,  52,  55, 
58,  366,  379,  382,  397,  401 

Sphenodon,  32 

spider,  259 

spinal    cord,    427,    428;    caliber    of, 

432,  433 ;  columns  of,  434 ;  length, 
428-431 ;    shape    of    cross-section, 

433,  434 

spinal   ganglia,  436,  437 
spinal  nerves,  427-434 
spinous  processes,   excessive  devel- 
opment of,   134 
spiny  ant-eater,  33 
spiracular    cartilage,    155 
spiracular  opening,  493,   494 
spiraculum,   155,  269 
spleen,  293,  363. 
Squalus,  388,  446 
squid,  499 
squirrel,  316 
squamosals,  82,  83,  147 
stapedial  artery,  159 
stapes,   159,  337,  495 
Stegocephali,  30,  31,  32,  44,  84,  230 
Stegosaurus,  433 
Stenson's   canal,  479 
sternebrae,  140 
sternum,    138 

sternum,    morphology   of,    139 
sternum  of  monotremes,   142 
stoma,  266 
stomach,  265,  290,  291,  292 


stomatodaeum,  259,  476 
stomato-pharyngeal    cavity,    266 
stratum  corneum,  77 
stratum    germinativum,    77 
stratum  lucidum,  77 
stratum  mucosum,  77 
striated  muscle,  189,  190 
sturgeon,  29 
stylo-hyal,  161,  284 
stylo-hyoid  ligament,    161 
styloid  process,   161,  284 
sub-cutaneous  lymph  sacs,  358 
subdural  space,  357 
sub-lingua,  284 
sublingual  glands,  285,  286 
submandibular  glands,  285,  286 
submaxillary    glands,    285,    286 
submucosa,  260 
subperitoneal  space,  357 
subvertebral   space,   357,   363 
sulci,  of  brain,  417 
sulcus  centralis,  417 
superciliary   structures,   505 
supra-clavicles,    174 
supra-cleithra,  174 
suprapericardial    bodies,  ,288 
Sus,  86,  489 

suspensorium  of  jaw,  155 
sweat-glands,    112,    113 
swine,  484,  491 
Sycandra,  50 

sympathetic   plexuses,   464 
sympathetic  system,  451,  463,  464, 
symphysis  pubic,   172 
syntropists,  244 
syrinx,  314. 


tactile  cells,  473 

tactile  corpuscles,  473 

tactile   sense,  distribution  of,  475 

tactile  spots,  473 

taania   ventriculi    quarti,   427 

taenise  chorioides,  419,  420 

tail,   130,   132,   134,   135 

tail,  nerves  of,  431,  432 

talon,  279 

Talpa,  277 

tapetum  nigrum,  423 

tapir,  394 

tarsal  cartilages,  504 

tarsal  glands,    113,   505 


INDEX 


571 


Tarsipes,  292 

tarsus: — nomenclature  of,  177-180; 
primitive  condition  of,  179;  su- 
pernumerary elements,  181,  182 

taste,  475,  476 

taste-beakers,  475 

taste  buds,  469,  471,  475 

Taxeopoda,   38 

teleosts,  30,  174,  287,  303,  305,  415, 
416,  419,  430,  486,  489 

teeth,  267,  271,  283.  (V.  also 
tooth)  : — evolution  of  shapes, 
278-280;  growth  of,  273,  274; 
kinds  of,  275;  of  birds,  84;  of 
selachians,  81 ;  origin  of,  81,  153, 
154,  271,  272;  parts  of,  273;  re- 
duction of,  277;  replacement  of, 
280-283 

tela  chorioidea,  419 

telencephalon,  411,  415-418,  443 

temperature  of  animals,  356 

tentorium,  417 

testes,  49,  367,  379,  383,  394,  39$ 

testes,  position  of,  393,  395 

thalami  optici,  425 

thecadont  articulation,  273 

thecse  of  jaws,  273 

thenar  pads,  91 

theories  of  vertebrate  ancestry : — 
from  annelids,  513-520;  from 
arachnoids,  522-525 ;  from  arche- 
type, 506-512;  from  articulates, 
512,  513;  from  insect,  506,  512; 
from  nemertean,  520-522;  from 
protochordata,  525-538 

theromorphs,  32,  44 

third   ventricle   of   brain,   411 

thoracic  duct,  357,  360 

thoracic  vertebrae,   130 

thymus  gland,  286,  290,  363 

thymus,  origin  of,  286-287 

thyreoid  cartilage,    160,  314 

thyreoid  gland,  286,  289,  290,  363 

thyreo-hyal,  284,  314 

Tillodontia,   37 

toad,  285,  290,  397,  420,  442 

tongue,  283-285 

tongue  bars,   of  Amphioxus,   290 

tonsils,   363 

tooth.    (V.  also  teeth) 

tooth  generations,  281 

tooth,  primitive  form  of,  274 

tooth,  structure  of,  80 


Tornaria,  536,  537 

tortoise-shell,    105 

trabeculae,  144 

trachea,  312 

trachea,  origin  of,  270 

tracheal  pieces,  160,  313 

tracheal   rings,  313,  314 

triconodont  dentition,  279 

trigonodont  dentition,  279 

trilobites,   19 

triradii,  91 

Triton,  19,  67,  313,  483 

tritubercular  theory,   278,  279 

truncus  arteriosus,  324 

trunk  vertebrae,  130 

tuba  auditiva,  269 

tuberculum  auriculi,  497 

tubular  glands,  97,  112 

tunica  dartos,  396 

Tunicata,  26,  260,  303,  529,  530,  531, 

532,  533,  535,  537,  538 
tunica   vaginalis   communis,  396 
tunica  vaginalis  propria,  396 
turtles,  33,   179,  201,  253,  331,  360, 

398,  399,  419,  431,  433,  483 
Tylopoda,  39 
tympanic  bone,  496 
tympanic  bulla,  496 
tympanic  cavity,  ossicles  of,  159 
tympanic  membranes,  494 
tympano-hyal,    161,  284 
tympanum,  493-496 
typical  vertebra  (Owen's),  507,  508, 

Tyson's  glands,  113 
U 

ultimo-branchial  bodies,  289 

umbilical  arteries,  70,  71 

umbilical  cord,  377 

umbilical   veins,  70,  71 

umbilicus,    347 

umbilicus,  relation  to  bladder,  378 

uncinate  processes,   136 

ungulates,  394,  417,  480,  496 

unstriated  muscle,  189 

ureter,  376,  383 

urethra,  377,  393,  394,  401 

urethral  glands,  401 

urinary  bladder,  377,  378 

urinary  organs,   369-378 

urinary  papilla,   374 


572 


INDEX 


urinary  system,  369-378 

urodeles,  31,  44,  174,  187,  201,  206, 
227,  229,  238,  247,  287,  288,  346, 
354,  355,  440,  442,  47i,  493,  5O3 

urogenital  sinus,  378,  387,  388,  393, 

394 

urogenital  system,  origin  of,  64,  65 
uropygeal  gland,   HI 
urostyle,  135 
Ursus,  87 

uterine  ligaments,  390 
uterine  mucous  membrane,  72 
uterus,    383,    387,    388,    389,    394; 

bicornis,     390;     bipartitus,     390; 

duplex,     390;     masculinus,     393; 

simplex,  390 

utriculo-saccular   canal,   487 
utriculus,  487. 


vagina,  387,  389,  390,  394 

vasa  aberrantia,  392 

vasa  efferentia,  381,  383,  385 

vascular  system,  65,  317-319 
Vas  deferens,  377.    (See  ductus  def- 
erens) 

vein  or  veins: — 318,  328;  abdom- 
inal, 342;  allantoic,  322,  346,  347, 
348;  anterior  cardinal,  321,  326, 
349,  350;  azygos,  344,  350,  351; 
cardinal,  321,  326,  341 ;  caudal, 
327,  341,  343 ;  Cuvierian  duct,  322, 
326,  346 ;  duct  of  Cuvier,  322,  326, 
346;  ductus  venosus  Arantii,  348, 
349;  external  jugular,  350;  hemi- 
azygos,  351;  hepatic,  323,  327; 
hepatic  portal,  324,  327,  342, 
344;  iliac,  326,  346,  358;  in- 
ternal jugular,  350;  jugular,  350, 
358;  lateral,  326,  346;  omphalo- 
mesenteric,  323,  347,  348;  ova- 
rian, 346;  portal,  323,  327;  post 
cava,  342,  344,  345,  349;  posterior 
cardinal,  321,  326,  343,  344,  345, 

349,  358;    pulmocutaneous,    354; 
renal,    345;    renales    advehentes, 
324,  327;  renales  revehentes,  324, 
327;   renal  portal,  324,  327,  341; 
spermatic,    346;    subclavian,    327, 

350,  358;   subintestinal,  340,  349; 
subintestinal  of  Amphioxus,  528; 


veins —  (  Continued} 

umbilical,  322,  346,  347,  348;  vena 
anonyma,  350,  352;  vena  cava 
anterior,  352;  vena  cava  pos- 
terior, 342,  344,  345,  349;  vitel- 
line,  320,  340,  347,  353;  yolk,  320, 
340,  347 

velum  palati,  271 

ventral  fins,   164 

ventral   nerves,   436 

ventricle,  324,  353-357 

ventricles  of  brain,  62,  406,  411,  412 

Vermes,  258 

vertebrae,  articulations  of,  131 

vertebrae,  development  of,  124-127 

vertebral    column,    development    of, 
124-127 

vertebral  column  of  birds,  132 

vetebral   column,   regional   differen- 
tiation of,  130 

vertebral    foramina,    138 

vertebrate  history,  sketch  of,   13-15 

vertebrates,  phylogenetic  tree  of,  28 

vesicles  of  brain,  411 

vesicula  prostatica,  393,  394 

vernix  caseosa,  95 

vibrissae,  472 

vidian  canal,  450 

villi,  chorionic,  70,  71 

visceral  mesoderm,  63,  66 

visceral  arches,  267 

visceral  muscles,  190,   192,   193 

visceral   skeleton,    122,    152-162 

visceral     skeleton,     metamorphoses 
of,  161 

vitreous  humor,  423,  498,  501 

vomero-nasal  cartilage,  483 

vomero-nasal  organ,  483 

voluntary  muscle,  65,  189,   190 


W 


Weber's  apparatus,  486 

whale,  165,  478,  491,  505 

white  blood  corpuscles,  318,  362 

white  matter,  428 

wish-bone,    141 

Wolffian  body,  373,  386 

Wolffian  duct,  64,  374,  375,  387,  392 

woodchuck,  297 

woodpecker,  283. 


INDEX  .573 

X  yolk  stalk,  69,  70 

Xenarthra,  37.  Z 

Y  zona  radiata,  52 

zonary  placenta,  72 

yolk,  52,  53,  69,  73  zygapophyses,   508 

yolk  sac,  69,  70,  319  zygote,  7,  48,  58. 


LAST  DATE 
CENTS 


883510 


2-57967 


BiOLOGY 

'.,nO     . 
3 

UNIVERSITY  OF  CALIFORNIA  LIBRARY 


